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Entered, according to Act of Congress, in the year 1844, 

In the Clerk's Office of the District Court of Connecticut. 


Stereotyped by 

45 Gold ffeMti r e w Yori. 


SOME years since, I announced to the pnblic an intention of 
preparing a series of text-books, in Natural Philosophy and As- 
tronomy, adapted, respectively, to Colleges, Academies, and 
Common Schools. A. Treatise on Natural Philosophy in two 
volumes, 8vo, and a Treatise on Astronomy in one volume, 8vo, 
a School Philosophy, and a School Astronomy, each in a duode- 
cimo volume, have long been before the public, and have pass- 
ed through numerous editions. Various engagements have pre- 
vented my completing, until now, the original plan, by adding 
a work of a form and price adapted to the primary schools, and 
in a style so easy and familiar, as to be suited to pupils of an 
earlier age than my previous works. 

In writing a book for the pupils of our Common Schools, or 
for the younger classes in Academies, I do not, however, con- 
sider myself as writing for the ignorant and uncultivated, but 
rather for those who have but little time for these studies, and 
who, therefore, require a choice selection of principles, of the 
highest practical utility, and desire the greatest possible amount 
of valuable information on the subjects of Natural Philosophy 
and Astronomy, in the smallest compass. The image which I 
have had constantly before me, is that of an intelligent scholar, 
of either sex, from twelve to sixteen years of age, bringing to the 
subject a mind improved by a previous course of studies, and a 
capacity of being interested in this new and pleasing depart- 
ment of knowledge. I have imagined the learner, after having 
fully mastered the principles explained in the first part, which 
treats of Natural Philosophy, entering upon Astronomy, in the 
second part, with a capacity much enlarged by what he has al- 
ready acquired, and with a laudable curiosity to learn the se- 
crets of the skies. I have imagined his teacher lending him oc- 
casional aid from a map of the stars, or a celestial globe, and 

stimulating as well as rewarding his curiosity, by pointing out to 
him the constellations. It is hoped, also, that most of the teach- 
ers who use this work, will have the still higher advantage of 
affording to youthful curiosity a view with which it is always 
delighted, that of the moon, planets, and stars, through a tel- 

I should deem myself incompetent to write a book like the 
present, if I had not been, myself, a teacher, first in a common 
school, and afterward in an academy or grammar school of the 
higher order. No one, in my judgment, is qualified to write text 
books in any department of instruction, who does not know, by 
actual experience, the precise state of mind of the pupils for whom 
he writes. Several years of experience in teaching the rudi- 
ments of knowledge, in early life, and the education of a large 
family at a later period, have taught me the devices by which 
the minds of young learners are to be addressed, in order that 
subjects at once new, and requiring some powers of reflection to 
understand them, may be comprehended with perfect clearness, 
and of course with lively pleasure. Children are naturally fond 
of inquiring into the causes of things. We may even go far- 
ther, and say, that they begin from infancy to interrogate nature 
in the only true and successful mode, that of experiment and 
observation. With the taper, which first fixes the gaze of the 
infant eye, the child commences his observations on heat and 
light. With throwing from him his playthings, to the great per- 
plexity of his nurse, he begins his experiments in Mechanics, 
and pursues them successively, as he advances in age, studying 
the laws of projectiles and of rotary motion in the arrow and the 
hoop, of hydrostatics in the dam and the water wheel, and pneu- 
matics in the wind-mill and the kite. I have in my possession 
an amusing and well-executed engraving, representing a family 
scene, where a young urchin had cut open the bellows to find the 
wind. His little brother is looking over his shoulder with inno- 
cent and intense curiosity, while the angry mother stands be- 
hind with the uplifted rod, and a countenance which bespeaks 
the wo that impends over the young philosopher. A more ju- 
dicious parent would have gently reproved the error ; a more en- 

lightened parent might have hailed the omen as indicating a 
Newton in disguise. 

It is earnestly hoped, that the Rudiments of Natural Philoso- 
phy and Astronomy, as much, at least, as is contained in this 
small volume, will be studied in every primary school in onr 
land. In addition to the intellectual and moral advantages, 
which might reasonably be expected from such a general diffu- 
sion of a knowledge of the laws of nature, and the structure of 
the universe, incalculable advantages would result to society 
from the acquaintance, which the laboring classes would thus 
gain, with the principles of the arts ; principles which lie at 
the foundation of their daily operations, fora "principle in sci- 
ence is a rule in art." Such a knowledge of philosophical prin- 
ciples, would suggest easier and more economical modes of per. 
forming the same labor ; it would multiply inventions and dis- 
coveries ; and it would alleviate toil by mingling with it a con- 
stant flow of the satisfaction which always attends a clear un- 
derstanding of the principles of the arts. 

Although this treatise is especially designed for schools, yet I 
would venture to recommend it to readers of a more advanced 
age, who may desire a concise and comprehensive view of the 
most important and practical principles of Natural Philosophy 
and Astronomy, comprising the latest discoveries in both these 
sciences. The part on Astronomy, especially, when compared 
with the sketches contained in similar works, may be found, 
perhaps, to have some advantages in the selection of points most 
important to be generally known in perspicuity of style and 
arrangement and in simplicity and fulness of illustration. It 
may, however, be more becoming for the author to submit this 
comparison to the judgment of the intelligent reader. 




INTRODUCTION. Grand Divisions of the Natural Sciences, 9 


Extension and Impenetrability Divisibility Porosity Com- 
pressibility Elasticity Indestructibility Attraction, - 19 


Motion in general Laws of Motion Center of Gravity Prin- 
ciples of Machinery, - - 24 


Pressure of Fluids Specific Gravity Motion of Fluids "Won- 
derful Properties combined in Water, 64 


Properties of Elastic Fluids Air Pump Common Pump Sy- 
phonBarometer Condenser Fire Engine Steam and 
its Properties Steam Engine, -- 84 


General Objects of the Science Extent, Density, and Temper- 
ature of ihe Atmosphere Its Relations to Water Rela- 
tions to Heat Relations to Fiery Meteors, - - . - - 106 


Vifaatory Motion Velocity of Sound Reflexion of Sound 
Musical Sounds Acoustic Tubes Stethoscope, - 119 


Definitions Conductors and Non-Conductors Attractions and 
Repulsions Electrical Machines Leyden Jar Electrical 
Light and Heat Thunder Storms Lightning Rods Ef- 
fects of Electricity on Animals, .......126 



Definitions Attractive Properties Directive Properties Vari- 
ation of the Needle Dip Modes of making Magnets, - 145 


Definitions Reflexion and Refraction Colors Vision Micro- 
scopes and Telescopes, ------- 15] 


Definitions Diurnal Revolutions, ------ 190 


Telescope Transit Instrument Astronomical Clock Sextant, 395 


Sidereal and Solar Days Mean and Apparent Time Horizon- 
tal Parallax Length of Twilight in Different Countries, - 202 


Distance Magnitude Quantity of Matter Spots Nature and 
Constitution Revolutions Seasons, - ... 208 


Distance and Diameter Appearances to the Telescope Moun- 
tains and Valleys Revolutions Eclipses Tides, - - 221 


General View Inferior Planets Superior Planets Planetary 
Motions, -------._. 235 


Description Magnitude and Brightness Periods Quantity of 
Matter Motions Prediction of their Returns Dangers, 2G2 


Number, Classification, and Distance of the Stars Different 
Groups and Varieties Nature of the Stars, and the System 
of the World, - - 271 





1. As in Geography we have a clearer understanding 
of particular countries, if we first learn the great divi 
ions of the globe, so we shall see more fully the pecu- 
liar nature of the sciences we are now to study, if we 
first learn into what distinct provinces the great empire 
of science is divided. 

To describe and classify the external appearances of 
things in nature, is the province of Natural History ; 
to explain the causes of such appearances, and of all 
the changes that take place in the material world, is 
the province of Natural Philosophy. The properties 
of bodies which are presented to the senses, such as 
form, size, color, and the like, are called external char- 
acters ; all events or occurrences in the material world, 
are called phenomena. Natural History is occupied 


ARTICLE 1. What is the province ol Natural History? Of Natu- 
ral Philosophy ? What properties of bodies are called the external 
What are phenomena ? With what is Natural History 

chwacters ? 


chiofly .^vjth ths external o-haracters of bodies, which 
it de'scwjtes and cla^>iaes,, Natural Philosophy, with 
phenomena, which it reduces under general laws. 
Ti.ius. tne natural , hi stoanu first observes and describes 
the external characters of animals, vegetables, and min- 
erals, and then classifies them, by arranging such as 
resemble each other in separate groups. The natu- 
ral philosopher, also, first observes and describes the 
phenomena of nature and art, and brings together such 
as are similar, under separate laws ; for example, the 
phenomena and laws of winds, of storms, of eclipses, 
and of earthquakes. 

2. We may form some idea of the method of classi- 
fication in Natural History, and of the investigation of 
general principles or laws in Natural Philosophy, by 
taking examples in each. The individual bodies that 
compose the animal, the vegetable, and the mineral 
kingdoms, are so numerous that, in a single life, we 
could make but little progress in acquiring a knowl- 
edge of them, if it were not in our power to collect into 
large groups, such as resemble each other in a greater 
or less number of particulars. When this is done, our 
progress becomes comparatively rapid ; for what we 
then learn respecting the group, will apply equally to 
all the individuals comprised in it. Hence, the various 
bodies in the several kingdoms of nature, are distribu- 
ted into classes, orders, genera, species, and varieties. 
Thus, those minerals which are like each other in 
having a certain well-known lustre, are collected to- 
gether into one CLASS, under the head of Metals, while 
others destitute of this peculiar character, but having 
certain other characters in common, are collected into 

chiefly occupied ? Ditto Natural Philosophy 1 Give an exam pie of the 
objects of the Natural Historian. Also of the Natural Philosopher. 
2. "Why is it necessary to classify the productions of nature ? How 
does such a classification make our progress more rapid 1 Into 
what are the various bodies in nature distributed ? 


another class, under the head of Earths.* But some 
metals, as lead and iron, easily rust, while others, as 
gold and silver, do not rust at all. Hence, metals are 
distributed into two ORDERS ; those which easily cor- 
rode being called base metals, and those which do not 
corrode, noble metals. But the members of each order 
have severally distinctive properties, which give rise 
to a further division of an order into GENERA. Thus, 
iron constitutes one genus and lead another, of the or- 
der of base metals. But of each of these genera there 
are several sorts, as wrought iron and cast iron, white 
lead and red lead. Each genus, therefore, is subdivi- 
ded into SPECIES, by grouping together such members 
of the same genus as resemble each other in several 
particulars. Finally, the individuals of each species- 
may differ from each other, and hence the 'species is 
still further divided into VARIETIES. Thus, Swedes 
iron and Russia iron, are varieties of the same species 
of the genus wrought iron, of the order of base metals. 
3. The knowledge we gain of any individual i>ody, 
depends upon the extent to which we carry the clas- 
sification of it. It is something to ascertain the class 
to which ir. belongs ; for example, that the body is a 
metal and not an earth. It is still more to learn to- 
what order of metals it belongs, as that it is one of the 
base and not one of the noble metals. We have ad- 
vanced still further when we have ascertained that it 
belongs to the genus iron, and not to that of lead. If 
we find that it is wrought and not cast iron, we ascer- 
tain the species ; and, finally, if we learn that it is 

* This example is given merely for the purpose of illustrating the method 
of classification, and not of showing the classification of minerals as actually 
a'dopted. This would be too technical for our present purpose. 

Give .an example of classification in the case of minerals. 

3. Upon what does the knowledge we acquire of any individual 
body depend 1 Show how we proceed from the class to the order, 
from the order to the genus, from the genus to the species, and from 
the species to the variety. 


Swedes and not Russia wrought iron, we determine* 
the variety. In regard to a body newly discovered, 
whether an animal, plant, or mineral, we may gene- 
rally discover very readily to what class and order it 
belongs, but it is usually more difficult to determine its 
exact species or variety. 

4. A clear understanding of the method of classifi- 
cation employed in Natural History, will aid us in 
learning the method of determining general principles, 
or laws, in Natural Philosophy. A law is the mode in 
which the powers of nature act ; and this is determined 
by the comparison of a great number of particular 
cases. Thus, when we have examined the directions 
of rays of light under a great variety of circumstan- 
ces, and always found them to be in straight lines, we 
say it is a law of light to move in straight lines. Laws 
are more or less extensive, according to the extent of 
phenomena they embrace. Thus, it is a law of the 
magnet that it attracts iron : it is a more extensive law 
(that of gravitation) that all bodies attract each other. 

5. The proper method of investigating any subject 
in Natural Philosophy, is, first, to examine with great 
attention all the facts of the case ; secondly, to clas- 
sify these, by arranging under the same heads, such 
as relate to the same things ; and, thirdly, to state the 
conclusions to which such a comparison of the phe- 
nomena leads us. These conclusions constitute the 
laws of that subject. Thus, if we apply heat to vari- 
ous bodies, and measure them before and after heating, 
we find in all cases that their size is enlarged. Hence 
we derive the law, that heat expands all bodies. If 
we expose solid bodies to a certain degree of heat, 

4. What is a law 1 How is it determined 1 How exemplified in 
the case of light 1 Show that laws may be more or less extensive. 

5. What is the proper method of investigating any subject in Natural 
Philosophy 1 What is the first step ? the second 1 -third 1 What 
do the conclusions constitute ? Give an example in the case of heat- 


they melt or become liquid, and liquids again are 
changed in the same way to vapor. Having observed 
these effects in a great number of individual cases, we 
lay it down as a law, that heat changes solids to fluids 
and fluids to vapors. By similar inquiries we ascer- 
tain all the laws of heat, which we perceive are, ac- 
cording to our definition, (Art. 4,) nothing more than 
the modes in which heat acts on various bodies. Laws 
or general principles like these, under one or another 
of which all the phenomena of the material world are 
reduced, constitute the elements of Natural Philos- 

6. The laws of nature, when once learned, are ap- 
plied to the explanation of the phenomena of nature 
or art, by a process somewhat similar to that of clas- 
sification in Natural History. It would afford a partial 
explanation of the motion of a steamboat on the water, 
to refer it to the general law of elastic force, which 
steam has in common with air, and several other nat- 
ural agents ; but it would be a more complete expla- 
nation to assign the particular mode in which the force 
acts upon the pistons, wheels, and other parts of the 
machinery. Science is a collection of general princi- 
ples or laws : Art, a system of rules founded on them. 
Arithmetic, so far as it explains the properties of num- 
bers, is a science : so far as it furnishes rules for the 
solution of problems, or for calculation, it is an art. 
A principle in science, therefore, is a rule in art. 

7. The term " Natural Philosophy" originally signi- 
fied, the study of nature in general. But as the objects 

What constitute the elements of Natural Philosophy "\ 

6. How are the laws of nature applied to the phenomena of na- 
ture and art ? What would be a partial explanation of the motion 
of a steamboat 1 What would oe a more complete explanation 1 
Distinguish between science and art. How far is arithmetic a sci- 
ence, and how far an art 1 What relation have the principles of sci- 
ence to the rules of art 1 

7. What did the term Natural Philosophy originally signify 1 



that fell under its notice were multiped, the field be- 
name too vast for one mimi. and it was divided into two 
parts what related to the earth h.'longod to Natural 
Philosophy, while the- stpn'v >ily hodies 

was erected into a separate dep under the head 

of Asti*0tlbig[iy. By whole of 

terrestrial natu. were fur- 

ther inultiplieri, presented too rone mind 

t<; explore, vnd Natural. Phi' ;ir as restricted to 

the investigation of il"; //-?//* re. while the de- 

scription and classification of , productions of the 
several kingdoms of nature, w^\ assigned to a distinct 
department undt-j u,i name of : lira) History. Still, 
it was a work tot- va-s- to take :<i of all the 'phenom- 
ena of nan : ir i :u;ate all the laws that 
govern them, ai 'bilosophy was again 
divided int : Mtjch&nical Phil -phy and Chemistry. 
M^chahii - ;! l^iilosophy relates , the phenomena and 
laws uf manses of mutter ; Chemistry, to the phenom- 
ena and la.\\vS of particles of matter. Mechanical 
Philosophy considers those effects only which are not 
attended by any change of nature, such as change of 
place, (or motion,; change of figure, and the like. 
Chemistry considers those effects which result from, 
the action of the p&rtidos of matter on each other, 
and which more or less change the nature of bodies, 
so as to make them something different from what 
they were before. Finally, it became too much for 
one class of laborers to investigate the changes of na- 
tu f> e : or constitution, which are constantly going on in 
every body in nature, and in every process, natural or 
;ul, and Chemistry was, therefore, restricted to 

Why was it divided into two parts ? What belonged to Natural 
Philosophy 1 What, to Astronomy 1 How was Natural Philosophy 
ptii.l lurtner divided 1 To whai was it restricted 1 What was assigned 
to Natural History ^ Into what was Natural Philosophy again divi- 
ded 1 To what, does Mechanical Philosophy relate 1 What Chemistry ? 
Wimt effects does Meshaaieal Philosophy consider 1 What Ghent- 


inanimate majer, while/ what. r^ites to i, 
was erected into a separate department undc 
of Physiology. 

8. 'Natural* History, moreover, found for i:-elf an 
empire too vast, in attempting to ctcscr'be'an'' clas^fy 
the external appearances of all thir.g in nature. 
Hence this study has 1 en successiv ) divided into 
various departments, the study of veg" \bles being re- 
ferred to Botany of auir- al> to"' Zo '^y ; of inanimate 
substances to Mineralogy. Still further subdivisions 
have been introduced i"to each of these branches uf 
Natural History, as :he o" ?ct-- embraced in it have 
multiplied. Thus, the stu< of that brarch of Zoolo- 
gy which relates to fi* ?e ias been erected into a 
separate department und. J he head of Ichthyology 
of birds into Ornith ogv ; and ct* insects Uito En- 

9. A division of tb studies which prelate to t 
world we inhabit, La : also ho.n made int< three ct 
partments, Geogrc*.ph~. , Geology, ami Metc ic-iogy ; aT 
objects on the surface of the earth being assigned 
Geography ; bematii the surface, to Goology ; an 
above the surface, to Meteorology. Of these, G 
raphy, in this extensive signification, present \\ 
largest field, since it comprehends, among (,,he^ M ii 
MAN and his works. 

10. Mechanical Philosophy is, strictl) -'aki - 
the branch of human kijovvledge which ( \\ 

pose to learn ; but it still retains the or' 1 
Natural Philosophy, though in a senso 

istry 1 How was Chemistry divided? T<' .1 ' 

what was assigned to Physiology ? 

8. Into what has Natural History been snrc^ssively u 
"What was referred to Botany? What to Zo< ' , ' A\ lat i Min- 
eralogy? What further subdivisions ijave b -n intri-du. mto 
each of these branches 1 

9 Into what three departments has all tenv-;i \\ nature btf-n di- 
vided 1 What isaasianed to Geography? what to GevlogyliUM, 
what to Meteorology ? Which presents l^ largest n'elu 1 


Tided, compared with its ancient signification. The 
complete investigation of almost any subject, either of 
nature or art, usually, in fact, enters the peculiar pro- 
vince of several kindred departments of science. For 
example, let us follow so simple a substance as bread, 
from the sowing of the grain to its consumption as 
food, and we shall find that the successive processes 
involve, alternately, the principles of Mechanical Phi- 
losophy, Chemistry, and Physiology. The ploughing 
of the field is mechanical and not chemical, because 
it acts on masses of matter, and produces no change 
of nature in the matter on which it operates, so as to 
make it something different from what it was before, 
but merely changes its place. For similar reasons the 
sowing of the grain is mechanical. But now a change 
occurs in the nature of the seed. By the process 
called germination, it sprouts and grows and becomes 
a living plant. As this is a change which takes place 
between the particles of matter, and changes the na- 
ture of the body, it seems, by our definition, to belong 
to Chemistry, and it would do so were not the changes 
those of living matter : that brings it under the head 
of Physiology. All that relates to the growth and 
perfecting of the crop is, in like manner, physiological. 
The reaping, carting, and threshing the wheat, are all 
mechanical processes, acting as they do on masses of 
matter, and producing no alteration of nature, but 
merely a change of place. The grinding and separa- 
tion of the grain into flour and bran, looks like a chem- 
ical process, because it reduces the wheat to particles, 
and brings out two new substances. We have, how- 
ever, only changed the figure and place. The grain 

10. What is strictly our subject 7 What other name does it still re- 
tain 1 What is true of the complete investigation of any subject in 
nature or art 1 How exemplified in the case of bread ? Why is the 
ploughing mechanical 1 Why is the sowing mechanical 1 Why is the 
germination physiological 1 How is it with the reaping, carting, and 
threshing 1 The grinding and manufacture into flour ? Making the 


consists of the same particles before and after grind- 
ing, and no new substance is really produced by the 
separation of the flour from the bran, for both were con- 
tained in the mixture, having the same nature before as 
after the separation. We next mix together flour, water, 
and yeast, to make bread, and bring it to the state of 
dough. So far the process is mechanical ; but now 
the particles of these different substances begin to act 
on each other, by the process called fermentation, and 
new substances are produced, not existing before in 
either of the ingredients, and the whole mass becomes 
something of a very different nature from either of the 
articles of which it was formed. Here then is a chem- 
ical change. Next we make the dough into loaves and 
place them in the oven by processes which are me- 
chanical ; but again heat produces new changes among 
the particles, and brings out a new substance, bread, 
which is entirely different in its nature both from the 
original 'ingredients and from dough. This change, 
therefore, is chemical. Finally, the bread is taken into 
the mouth, masticated, and conveyed to the stomach 
by mechanical operations ; but here it is subjected to 
the. action of the principle of life that governs the ani- 
mal system, and therefore again comes under the pro- 
vince of physiology. 

11. The distinction between terms, which are apt 
to be confounded with each other, may frequently be 
expressed by single words or short phrases, although 
they may not convey full and precise definitions. The 
following are examples : History respects facts ; Phi- 
losophy, causes ; Physics, matter ; Metaphysics, mind ; 
Science, general principles ; Art, rules and instruments. 
Physical laws are modes of action ; moral and civil 

bread ? Its fermentation 1 Forming into loaves and p lacing in tha 
oven ? The baking ? eating 1 the final change in the stomach 1 

11. What does History respect 1 What Philosophy '{Physics ? 
Metaphysics r { Science? Art 1 What are physical and what 

moral laws 1 What is the province of Natural, and -what that of 


laws, rules of action. The province of Natural Philos- 
ophy is the material world ; that of Moral Philosophy 
is the soul. Mechanical effects result from change of 
place or figure ; Chemical, from change of nature. 
Chemical changes respect inanimate matter ; Physio- 
logical, living matter. 

12. Mechanical Philosophy takes account of such 
properties of matter only as belong to all bodies what- 
soever, or of such as belong to all bodies in the same 
state of solid, fluid, or aeriform. These are few in 
number compared with the peculiar properties of indi- 
vidual bodies, and the changes of nature which they 
produce on each other, all of which belong to Chemis- 
try. Chemistry, therefore, is chiefly occupied with 
matter ; Natural Philosophy, with motion. The lead- 
ing subjects of Natural Philosophy are 

1. MATTER its general properties. 

2. MECHANICS the doctrine of Motion. 

3. HYDROSTATICS the doctrine of Fluids in the form 
of water. 

4. PNEUMATICS the doctrine of Fluids in the form 
of air. 

5. METEOROLOGY the Atmosphere and its phe- 

6. ACOUSTICS the doctrine of Sound. 



9. OPTICS the doctrine of Light. 

Moral Philosophy! From what do mechanical effects result! 
from what chemical 1 What do chemical changes respect, and 
what physiological ! _ 

12. Of wh at properties does Mechanical Philosophy take account 1 
With what is chemistry chiefly occupied ! witn what is Natural 
Philosophy 1 Enumerate the leading subjects of Natural Phi- 




13. All matter has at least two properties Exten- 
sion and Impenetrability. The smallest conceivable 
portion of matter occupies some portion of space, and 
has length, breadth, and thickness. Extension, there- 
fore, belongs to all matter. Impenetrability is the 
property by which a portion of matter excludes all 
other matter from the space which it occupies. Thus, 
if we drop a bullet into water, it does not penetrate 
the water, it displaces it. The same is true of a nail 
driven into wood. These two properties of matter are 
all that are absolutely essential to its existence ; yet 
there are various other properties which belong to 
matter in general, or at least to numerous classes of 
bodies, more or less of which are present in all bodies 
with which we are acquainted. Such are Divisibility, 
Porosity, Compressibility, Elasticity, Indestructibility, 
and Attraction. Matter exists in three different states, 
of solids, liquids, and gases. These result from its 
relation to heat ; and the same body is found in one or 
the other of these states, according as more or less 
heat is combined with it. Thus, if we combine with 
a mass of ice a certain portion of heat, it passes from 
the solid to the liquid state, forming water ; and if we 
add to water a certain other portion of heat, it passes 
into the same state as air, and becomes steam. Chem- 
istry makes known to us a great number of bodies in 

13. What are the two essential properties of matter 1 Why does 
extension belong to all matter 1 Define impenetrability, and give 
an example. What other properties belong to matter 1 In what 
three diflerent states does matter exist 1 How exemplified in wa- 


the aeriform state, called gases, arising from the union 
of heat with various kinds of matter. The particles 
which compose water, for example, are of two kinds, 
oxygen and hydrogen, each of which, when united 
with heat, forms a peculiar kind of air or gas. 

14. Matter is divisible into exceedingly minute parts. 
A leaf of gold, which is about three inches square, 
weighs only about the fifth part of a grain, and is only 
the 282,000th part of an inch in thickness. Soap 
bubbles, when blown so thin as to display their gaudy 
colors, are not more than the 2,000,000th of an inch 
thick ; yet every such film consists of a vast number 
of particles. The ultimate particles of matter, or 
those which admit of no further division, are called 
atoms. The atoms of which bodies are composed are 
inconceivably minute. The weight of an atom of 
lead is computed at less than the three hundred bil- 
lionth part of a grain. Animalcules (insects so small 
as to be invisible to the naked eye, and seen only by 
the microscope) are sometimes so small that it would 
take a million of them to amount in bulk to a grain of 
sand ; yet these bodies often have a complete organi- 
zation, like that of the largest animals. They have 
numerous muscles, by means of which they often 
move with astonishing activity ; they have a digestive 
system by which their nutriment is received and ap- 
plied to every part of their bodies ; and they have 
numerous vessels in which the animal fluids circulate. 
What must be the dimensions of a particle of one of 
these fluids [ 

15. A large portion of the volume of all bodies con- 
sists of vacant spaces, or pores. Sponge, for example, 
exhibits its larger pores distinctly to the naked eye. 

ter *? What are bodies in the state of air called 1 What agent 
maintains matter in the state of gas 1 

14. Divisibility. Examples in gold leaf soap bubbles. What are 
atoms 1 weight of an atom of lead 1 ? What are animalcules 1i 
Show the extreme minuteness of their parts 


But it also has smaller pores, of which the more solid 
matter of the sponge itself is composed, which are 
usually so small as to be but faintly discernible to the 
naked eye. The cells which these parts compose are 
separated by a thin fibre, which itself exhibits to the 
microscope still finer pores ; so that we find in the 
same body several distinct systems of pores. Even 
the heaviest bodies, as gold, have pores, since water, 
when enclosed in a gold ball and subjected to strong 
pressure, may be forced through the sides. Most an- 
imals and vegetables consist in a great degree of mat- 
ter that is exceedingly porous, leaving abundant room 
for the peculiar fluids of each to circulate. Thus, a 
thin slip or cross section of the root or small limb of a 
tree, exhibits to the microscope innumerable cells for 
the circulation of the sap. 

16. All bodies are more or less compressible, or may 
be reduced by pressure into a smaller space. Bodies 
differ greatly in respect to this property. Some, as 
air or sponge, may be reduced to a very small part of 
their ordinary bulk, while others, as gold and most 
kinds of stone, yield but little to very heavy pressures. 
Still, columns of the hardest granite are found to un- 
dergo a perceptible compression when they are made 
to support enormous buildings. Water and other 
liquids strongly resist compression, but still they yield 
a little when pressed by immense forces. 

17. Many bodies, after being compressed or extended, 
restore themselves to their former dimensions, and hence 
are called elastic. Air confined in a bladder, a sponge 
compressed in the hand, and India-rubber drawn out, 
are familiar examples of elastic bodies. If we drop 

15. Porosity Example in sponge. What proof is there that 
gold is porous 1 How do we learn that animal and vegetable mat- 
ter is porous 1 

16. Compressibility. How do bodies differ in this respect . 1 What 
bodies easily yield to pressure 1 what yield little 1 How is it with 
granite 1 with water 1 



Fig. 1. 

on the floor a ball of yarn, or of ivory or glass, it re- 
bounds, being more or less elastic ; whereas, if we do 
the same with a ball of lead, it falls dead without re- 
bounding, and is therefore non-elastic. When a body 
perfectly recovers its original 
dimensions, it is said to be 
perfectly elastic. Thus, air is 
perfectly elastic, because it 
completely recovers its former 
volume, as soon as the corn- 
pressing force is removed, 
" and hence resists compression 
with a force equal to that 
which presses upon it. Wood, 
when bent, seeks to recover 
itself on account of its elasti- 
city ; and hence its use in the 
bow and arrow, the force with 
which it recovers itself being 
suddenly imparted to the ar- 
row through the medium of 
the string. 

18. Matter is wholly indestructible. In all the chan- 
ges which we see going on in bodies around us, not a 
particle of matter is lost ; it merely changes its form ; 
nor is there any reason to believe that there is now a 
particle of matter either more or less than there was 
at the creation of the world. When we boil water 
and it passes to the invisible state of steam, this, on 
cooling, returns again to the state of water, without 
the least loss ; when we burn wood, the solid matter 
of which it is composed passes into different forms, 

17. Elasticity. Give examples. Show the difference bet'veen 
balls of ivory and lead. When is a body perfectly elastic 1 Give 
an example. Explain the philosophy of t'he bow and arrow. 

18. Indestructibility. Is matter ever annihilated or destroyed 1 
What becomes of water when boiled, and of wood when burned 1 


some into smoke, some into different kinds of airs, or 
gases, some into steam, and some remains behind in 
the state of ashes. If we should collect all these 
various products, and weigh them, we should find the 
amount of their several weights the same as that of 
the body from which they were produced, so that no 
portion is lost. Each of the substances into which 
the wood was resolved, is employed in the economy 
of nature to construct other bodies, and may finally 
reappear in its original form. In the same manner, 
the bodies of animals, when they die, decay and seem 
to perish ; but the matter of which they are composed 
merely passes into new forms of existence, and reap- 
pears in the structure of vegetables or other animals. 
19. All matter attracts all other matter. This is 
true of all bodies in the Universe. In this extensive 
sense, attraction is called Universal Gravitation. In 
consequence of the attraction of the earth for bodies 
near it, they fall toward it, arid this kind of attraction 
is called Gravity. Several distinct cases of this prop- 
erty occur also among the particles of matter. That 
which unites particles of the same kind (as those of a 
musket ball) in one mass, is called Aggregation ; fliat 
which -unites particles of different kinds, forming a 
compound, (as the particles of flour, water, and yeast 
in bread,) is Affinity. The term Cohesion is used to 
denote simply the union of the separate parts that 
make up a mass, without considering whether the par- 
ticles themselves are simple or compound. Thus the 
grains which form a rock of sandstone, are united by 
cohesion. Magnetism and electricity also severally 
endue different portions of matter with tendencies 
either to attract or repel each other, which are called, 

What becomes of the bodies of animals when they die 1 
19. Attraction. How extensive 1 What is it called when ap- 
plied to all the bodies in the universe 1 Why do bodies fall toward 
the earth 1 What is this kind of attraction called 1 What is aggrega- 
tion 1 affinity 1 cohesion 1 Give an example of each. Wnai are 


respectively, Magnetic and Electric attractions. Te- 
nacity, or that force by which the particles of matter 
hang together, is only a form of cohesion. Of all 
known substances, iron wire has the greatest tenacity. 
A number of fine wires bound together constitute what 
is called a wire cable. These cables are of such pro- 
digious strength that immense bridges are sometimes 
Fig. 2. 

suspended by them. The Menai bridge, in Wales, 

one of the greatest works in modern times, is thus 

supported at a great height, although it weighs toward 
two thousand tons. 




20. MECHANICS, or tfre DOCTRINE OF MOTION, is that 
part of Natural Philosophy which treats of the laws of 
equilibrium and motion. It considers also the nature 
of the forces which put bodies in motion, or which 
maintain them either in motion, or in a state of rest or 
equilibrium. The great principles of motion are the 

magnetic and electric attractions 1 Define tenacity. What sub- 
stance has the greatest 1 How employed in bridges 1 
20. Define mechanics. What are those agents called which pat 


same everywhere, being applicable alike to solids, 
liquids, and gases ; to the most common objects around 
us, and to the heavenly bodies. The science of Me- 
chanics, therefore, comprehends all that relates to the 
laws of motion ; to the forces by which motion is pro- 
duced and maintained ; to the principles and construc- 
tion of all machines ; and to the revolutions of the 
heavenly bodies. 

SECTION 1. Of Motion in general. 

21. Motion is change of place from one point of 
space to another. It is distinguished into real and 
apparent ; absolute and relative ; uniform and variable. 
In real motion, the moving body itself actually changes 
place ; in apparent motion, it is the spectator that 
changes place, but being unconscious of his own mo- 
tion, he refers it to objects without him. Thus, when 
we are riding rapidly by a row of trees, these seem 
to move in the opposite direction ; the shore appears 
to recede from the sailor as he rapidly puts to sea ; 
and the heavenly bodies have an apparent daily motion 
westward, in consequence of the spectator's turning 
with the earth on its axis to the east. Absolute mo- 
tion is a change of place from one point of space to 
another without reference to any other body : Relative 
motion is a change of position with respect to some 
other body. Two bodies may both be in absolute mo- 
tion, but if they do not change their position with 
respect to each other, they will have no relative mo- 
tion, or will be relatively at rest. The men on board 
a ship under sail, have all the same absolute motion, 

bodies in motion or keep them at rest 1 How extensively do the 
great principles of motion prevail ] What does the science of me- 
chanics comprehend 1 

21. Define motion. Into what varieties is it distinguished 1 Ex- 
plain the difference between real and apparent motion. Give ex- 
amples of apparent motion. Distinguish between absolute and re- 
lative motion. Example in the case of persons on board a ship 


and so long as they are still, they have no other ; but 
whatever changes of place occur among themselves, 
give rise to relative motions. If two persons are 
travelling the same way, at the same rate, whether in 
company or not, they have no relative motion ; if one 
goes faster than the other, the latter has a relative 
motion backward equal to the difference of their rates ; 
and if they are travelling in opposite directions, their 
relative motion is equal to the sum of both their mo- 
tions. A body moves with a uniform motion when it 
passes over equal spaces in equal times ; with a vari- 
able motion, when it passes over unequal spaces in 
equal times. If a man walks over just as many feet 
of ground the second minute as the first, and the third 
as the second, his motion is uniform ; but if he should 
walk thirty feet one minute, lorty the next, and fifty 
the next, his motion would be variable. 

22. Force is any thing that moves, o,r lends to move a 
body. The strength of an animal exerted to draw a 
carriage, the impulse of a waterfall in turning a wheel, 
and the power of steam in moving a steamboat, are sev- 
erally examples of a force. A weight on one arm of 
a pair of steelyards, in equilibrium with a piece of 
merchandise, although it does not move, but only tends, 
to move the body, is still a force, since it would pro- 
duce motion were it not counteracted by an equal force. 
The quantity of motion in a body is called its momen- 
tum. Two bodies of equal weight, as two cannon- 
balls, will evidently have twice as much motion as 
one ; nor would it make any difference if they were- 
united in one mass, so as to form a single body of 
twice the weight of the separate balls ; the quantity of 
motion would be doubled by doubling the mass, while 
the velocity remained the same. Again, a ball that 

in the case of travellers 1 When does a body move with uniform 
jnotion 1 When with variable motion *? Example. 
22. Define force. &amples. Wha,t is momentum 1 Upon what 


moves twice as fast as before, has twice the quantity 
of motion. Momentum therefore depends upon two 
things the velocity and quantity of matter. A large 
body, as a ship, may have great momentum with a slow 
motion ; a small body, as a cannon-ball, may have 
great momentum with a swift motion ; but where great 
quantity of matter (or mass) is united with great swift- 
ness, the momentum is greatest of all. Thus a train 
of cars on a railroad moves with prodigious momen- 
tum ; but the planets in their revolutions around the sun, 
with a momentum inconceivably greater. 

23. To the eye of contemplation, the world presents 
a scene of boundless activity. On the surface of the 
earth, hardly any thing is quiescent. Every tree is 
waving, and every leaf trembling ; the rivers are run- 
ning to the sea, and the ocean itself is in a state of 
ceaseless agitation. The innumerable tribes of ani- 
mals are in almost constant motion, from the minutest 
insect to the largest quadruped. Amid the particles of 
matter, motions are unceasingly going forward, in as- 
tonishing variety, that are effecting all the chemical 
and physiological changes to which matter is constantly 
subjected. And if we contemplate the same subject 
on a larger scale, we see the earth itself, and all that 
it contains, turning with a steady and never ceasing 
motion around its own axis, wheeling also at a vastly 
swifter rate around the sun, and possibly accompany- 
ing the sun himself in a still grander circuit around 
some distant center. Hence, almost all the phenom- 
ena or effects which Natural Philosophy has to inves- 
tigate and explain are connected with motion and de- 
pendent on it. 

two things does it depend *? What union of circumstances produces 
great momentum 1 Example. 

23. What proofs of activity do we see in nature 1 Give examples 
in the vegetable kingdom in the animal among the particles of 
matter and among the heavenly bodies. Upon what are almost 
all the phenomena of Natural Philosophy dependent 1 


SEC. 2. Of the Laws of Motion. 

24. Nearly all the varieties of motion that fall with 
in the province of Mechanical Philosophy, have been 
reduced to three great principles, called the Laws of 
Motion. We will consider them separately. 

FIRST LAW. Every body will persevere in a state of 
rest, or of uniform motion in a straight line, until com- 
pelled by some force to change its state. This law 
contains four separate propositions ; first, that unless 
put in motion by some external force, a body always 
remains at rest ; secondly, that when once in motion 
it always continues so unless stopped by some force ; 
thirdly, that this motion is uniform ; and fourthly, that 
it is in a straight line. Thus, if I place a ball on a 
smooth sheet of ice, it will remain constantly at rest 
until some external force is applied, having no power 
to move itself. I now apply such force and roll it ; 
being set in motion, it would move on forever were 
there no impediments in the way. It will move uni- 
formly, passing over equal spaces in equal times, and 
it will move directly forward in a straight course, turn- 
ing neither to the right hand nor to the left. This 
property of matter to remain at rest unless something 
moves it, and to continue in motion unless something 
stops it, is called Inertia. Thus the inertia of a steam- 
boat opposes great resistance to its getting fully into 
motion ; but having once acquired its velocity, it con- 
tinues by its inertia to move onward after the engine is 
stopped, until the resistance of the water and other 
impediments destroy its motion. The planets continue 
to revolve around the sun for no other reason than 
this, that they were put in motion and meet with noth- 
ing to stop them. Whenever a horse harnessed to a 
carriage starts suddenly forward, he breaks his traces, 

24. To how many great principles have all the varieties of motion 
been reduced 1 What are they called 1 State the first law. Enu- 
merate the four propositions contained in this law. Example. 



because the inertia of the carriage prevents the sudden 
motion being instantly propagated through its mass, 
and the force of the horse being all expended on the 
traces, breaks them. On the other hand, if a horse 
suddenly stops, when on a run, the rider is thrown 
over his head ; for having aco 1 uired the full motion of 
the horse, he does not instantly lose it, but, on ac- 
count of his inertia, continues to move forward after 
the force that put him in motion is withdrawn. This 

Fig. 3. 

principle is pleasingly illus- 
trated in what is called the 
doubling of the hare. A hare 
closely pursued by a grey- 
hound, starts from A, and when 
he arrives at C, the dog is - 
hard upon him ; but the hare 
being a lighter animal than 
the dog, and having of course 
less inertia, turns short at C 
and again at E, while the dog 
cannot stop so suddenly, but goes further round at 
D and also F, and thus the hair outruns him. Put 
a card of pasteboard across a couple of wine glass- 
es, and two sixpences di- 
rectly over the glasses, 
as in the figure ; then 
strike the edge of the 
card at A a smart blow, 
and the card will slip 
off and leave the money 
in the glasses. The 
coins, on account of their inertia, do not instantly 
receive the motion communicated to the card. If the 
blow, however, be gentle, all will go off together. 

Fig. 4. 

"What is inertia 1 Example in a steamboat in the planets in a 
horse in the doubling of a hare and in the card and com. 



Fig. 5. 



25. The first law of motion also asserts, that all 
moving bodies have a tendency to move in straight 
lines. We see, indeed, but few examples of such 
motions either in nature or art. If we throw a ball 
upward, it rises and falls in a curve ; water spouting 
into the air does the same ; rivers usually run and 
trees wave in curves ; and the heavenly bodies re- 
volve in apparent circles. Still, when we attentively 
examine each of these cases, and every other case of 
motion in curves, we find one or more forces opera- 
ting to cause the body to deviate from a 
straight line. When such cause of de- 
. viation is removed, the body immedi- 
ately resumes its progress in a straight 
line. This effort of bodies, when mov- 
ing in curves, to proceed directly for- 
ward in a straight line, is called the 
Centrifugal Force. If we turn a grind- 
stone, the lower part of which dips into 
water, as the velocity increases the 
water is thrown off from the rim in 
straight lines which touch the rim and 
are therefore called tangents* to it ; and 
it is a general principle, that when bodies 
free to move, revolve in curves about a 
center, they have a constant tendency to 
fly off in straight lines, which are tan- 
|c gents to the curves. We see this princi- 
ple exemplified in giving a rotary mo- 
tion to a pail or basin of water. The 
liquid first rises on the sides of the vessel, and if the 
rapidity of revolution be increased, it escapes from 

* A line is said to be a tangent to a curve, when it touches the curve, but 
does not cut it. 

25. Are the motions observed in the natural world, usually per- 
formed in straight or in curved lines 1 Why then is it said that bod- 
ies naturally move in straight lines 1 What is this effort to move in 


the top in straight lines which are tangents to the rim 
of the vessel. If we pass a cord through a staple in 
the ceiling of a room, and bringing down the two 
ends, attach them to the ears of a pail containing a 
little water, (suspending the vessel a few feet above 
the floor,) and then, applying the palms of the hands 
to the opposite sides of the pail, give it a steady rotary 
motion, the water will first rise on the sides of the 
vessel and finally be projected from the rim in tan- 
gents. The experiment is more striking if we suffer 
the cord to untwist itself freely, after having been 
twisted in the preceding process. 

26. SECOND LAW. Motion, or change of motion, 
is proportioned to the force impressed, and is produced 
in the line of direction in which that force acts. First, 
the quantity of motion, or momentum, is proportioned 
to the force applied. A double blow produces a 
double velocity upon a given mass, or the same velo- 
city upon twice the mass. Two horses applied with 
equal advantage to a load, will draw twice the load 
of one horse. It follows also from this law, that every 
force applied to a body, however small that force may 
be, produces some motion. A stone falling on the 
earth moves it. This may seem incredible ; but if 
we suppose the earth divided into exceedingly small 
parts, each weighing only a pound for example, then 
we may readily conceive how the falling stone would 
put it in motion. Now the effect is not lost by being 
expended on the whole earth at once ; the momen- 
tum produced is the same in both cases ; but in pro- 
portion as the quantity of matter is increased the 
velocity is diminished, and it would be as much less 

straight lines called 1 Example in a grindstone in a suspended 
vessel of water. 

26. What is the second law of motion 1 Show that the quantity 
of motion is proportioned to the force applied. Explain now the 
smallest force produces some motion. 


as the weight of the whole earth exceeds one pound. 
It would therefore be inappreciable to the senses, but 
still capable of being expressed by a fraction, and 
therefore a real quantity. " A continual dropping 
wears away stone." Each drop, therefore, must con- 
tribute something to the effect, although too small to 
be perceived by itself. 

27. Secondly, motion is produced in the line of 
direction in which the force is applied. If I lay a 
ball on the table and snap it with my thumb and finger, 
it moves in different directions according as I change 
the direction of the impulse ; and this is conformable 
to all experience. A single force moves a body in 
its own direction, but two forces acting on a body at 
the same time, move it in a line that is intermediate 
between the two. Thus, if I place a small ball, as a 
marble, on the table, and at the same moment snap it 
with the thumb and finger of each hand, it will not 
move in the direction of either impulse, but in a line 
between the two. A more precise consideration of 
this case has led to the following important law : 

If a body is impelled by two forces which may be re- 
presented in quantity and direction by the two sides of 
a parallelogram, it will describe, the diagonal in the same 
time in which it would have described each of the sides 
separately, by the force acting parallel to that side. 

Thus, suppose the parallelogram A B C D, repre- 
sents a table, of which the side A B is just twice the 
length of A D. I now place the ball on the corner A, 
and nail a steel spring (like a piece of watch spring) 
to each side of the corner, so that when bent back it 
may be sprung upon the ball, and move it parallel to 
the edge of the table. I first spring each force sep- 
arately, bending back that which acts parallel to the 

27. Show that motion is in the tine of direction of the force. 
How does a single force move a body 1 How do two forces move 
if? Ilecite the law represented in figured, and explain the figure. 



longer side so much further than the other, that the 
ball will move over the two sides in precisely the 
same time, sup- j^g. g 

pose two sec- Jy 
onds. I now let 
off the springs 
on the ball at 
the same in- 
stant, and the 
ball moves a- 
cross the table,? 
from corner to 
corner, in the 
same two sec- 
onds. It is not necessary that the parallelogram 
should be right-angled' like a table. The effect will 
be the same at whatever angle the sides of the paral- 
lelogram meet. 

28. If I take a triangular board instead of the table, 
and fix three springs at one corner, so as to act paral- 
lel to the three sides of the board, and give each 
spring a degree of strength proportioned to the length 
of the side in the direction of which it acts, and then 
let all those springs fall upon the ball at the same in- 
stant, the ball will remain at rest. This fact is ex- 
pressed in the following proposition : 

If three forces, represented in quantity and direction 
by the three sides of a triangle, act upon a body at the 
same time, it will be kept at rest. 

A kite is seen to rest in the air on this principle, 
being in equilibrium between the force of gravity 
which would carry it toward the earth, that of the 
string, and that of the wind, which severally act in 
the three directions of the sides of a triangle, and 

28. What is the effect of three forces, represented in quantity and 
direction by the three sides of a triangle 1 How does a kite exem- 
plify this principle 1 Is the principle confined to three directions 1 



B Fig. 7. 

neutralize each other. Nor is the principle confined 
to three directions merely, but holds good for a poly- 
gon of any number of sides. For example, a body 
situated at A, and acted upon by five forces repre- 
sented in quantity 
and direction by 
the five sides of 
the polygon, (Fig. 
7,) would remain 
at rest. If the for- 
ces were only four, 
corresponding to 
all the sides of the 
figure except the 
last, EA, then the 
body would de- 
scribe this side in 
the same time in 
which it would de- 
forces acting sepa- 


of the sides 

by the 

scribe each 

29. Simple motion is that produced by one force ; 
compound motion, that produced by the joint action of 
several forces. Strictly speaking, we never witness 
an example of simple motion ; for when a ball is 
struck by a single impulse, although the motion is 
simple relatively to surrounding bodies, yet the ball 
is at the same time revolving with the earth on its' 
axis and around the sun, and subject perhaps to innu-- 
merable other motions. Although all bodies on the 
earth are acted on at the same moment by many for- 
ces, and therefore it is difficult or even impossible to* 
tell what is the line each describes in space under 

Case of a polygon of five sides. "Where only four forces are ap- 
plied, how will the body move 7 

29. What is simple, and what compound motion *? Do we ever 
witness simple motions in nature 1 Example. When a force is 


their joint action, yet each individual force produces 
precisely the same change of direction in the body 
as though it were to act alone. If it acts in the same 
direction in which the body is moving, it will add its 
own amount ; if in the opposite direction, it will sub- 
tract it ; if sidewise, it will turn the body just as far 
to the right or left in a given time, as it would have 
done had it been applied to the body at rest. Thus, if 
while a body is moving Fi g 

from A to B, (Fig. 8,) it c D 

be struck by a force in the 
direction of AC, it will 
reach the line CD, in the 
same time in which it 
would have done had it 
been subject to no other 
force. It will, however, 
reach that line in the point 

D instead of C. When a A B 

man walks the decks of a ship under sail, his motions 
are precisely the same with respect to the other objects 
on board, as though the ship were at rest ; but the line 
which he actually describes under the two forces is 
very different. 

30. Instances of this diagonal motion are con- 
stantly presented to our notice. In crossing a river, 
the boat moves under the united impulses of the oars 
and the current, and describes the diagonal whose 
sides are proportional to the two forces respectively. 
Equestrians sometimes exhibit feats of horsemanship 
by leaping upward from the horse while running, and 
recovering their position again. They have, in fact, 

applied to a body in motion, what is the effect 1 Explain from Fi 
8. Case of a man walking the deck of a ship, 

30. Examples of diagonal motion. A boat crossing a 
EquefctriansT Two men ift a boat togsing $ ball^-i 


only to rise and fall as they would do were the horse 
at rest ; for the forward motion which the rider re- 
tains by his inertia, during the short interval of his 
ascent and descent, carries him onward, so that he 
rises in one diagonal and falls in another. Two men 
sitting on opposite sides of a boat in rapid motion, will 
toss a ball to each other in the same manner as though 
the boat were at rest ; but the actual movement of the 
ball will be diagonal. Rowing, itself, exemplifies the 
same principle ; for while one oar would turn the 
boat to the left and the other to the right, it actually 
moves ahead in the diagonal between the two direc- 

31. When, of two motions impressed upon a body, 
one is the uniform motion which results from an im- 
pulse, and the other is produced by a force which acts 
continually, the path described is a curve. Thus, 
when we shoot an arrow into the air, the impulse given 
by the string tends to carry it forward uniformly in a 
straight line ; but gravity draws it continually away 
from that line, and makes it describe a curve. In 
the same manner the planets are continually drawn 
away from the straight lines in which they tend to 
move, by the attraction of the sun, and are made to 
describe curved orbits about that body. 

32. THIRD LAW. When bodies act on each other, 
action and reaction are equal, and in opposite directions. 
The meaning of this law is, that when a body imparts 
a motion in any direction, it loses an equal quantity 
of its own that no body loses motion except by im- 
parting an equal amount to other bodies that when a 
body receives a blow it gives to the striking body an 
equal blow that when one body presses on another it 
receives from it an equal pressure that when one body 

31. Under what two forces will a body describe a curve 1 Exam- 
ples An arrow The planets. 

32. Give the third law of motion. Explain its meaning. Exam- 


attracts or repels another, it. is equally attracted or re- 
pelled by the other. If a steamboat should run upon 
a sloop sailing in the same direction with a slower 
motion, it might drive it headlong without experien- 
cing any great shock itself; still its own loss of motion 
would be just equal to that which it imparted to the 
sloop, but being distributed over a quantity of matter 
so much greater, the loss might be scarcely percep- 
tible. If a light body, as the wad of a cannon, were 
fired into the air, it would be stopped by the resistance 
of the air ; but its own motion would be lost only as it 
imparted the same amount to the air, and thus might 
be sufficient, on account of the lightness of air, to set 
a large volume in motion. When the boxer strikes his 
adversary, he receives an equal blow from the reaction 
of the part struck ; but receiving it on a part of less 
sensibility, he is less injured by it than his adversary 
by the blow inflicted on him. One who falls from an 
eminence on a bed of down, receives in return a resist- 
ance equal to the force of the fall, as truly as one who 
falls on a solid rock ; but, on account of the elasticity 
of the bed, the resistance is received gradually, and is 
therefore distributed more uniformly over the system. 
A boatman presses against the shore, the reaction of 
which sends the boat in the opposite direction. He 
strikes the water with his oar backward, and the 
boat moves forward. The fish beats the water with 
his tail, first on one side and then on the other, and 
moves forward in the diagonal between the two reac- 
tions. The bird beats the air with her wings, and the 
resistance carries her forward in the opposite direc- 
tion. All attractions likewise are mutual. The iron 
attracts the magnet just as much as the magnet attracts 
the iron. The earth attracts the sun just as much as 
the sun attracts the earth. In all these cases the mo- 

ptes of a steamboat running upon a sloop a wad fired into the air^-a 

boxer falling upon a feather bed a boatman a bird attractions. 




Fig. 9. 

mentum or quantity of motion in the smaller and the 
larger body, is the same. Thus, when a small boat 
is drawn by a rope toward a large ship, the ship 
moves toward the boat as well as the boat toward the 
ship, and with the same momentum ; but the space 
over which the ship moves is as much less than that 
of the boat, as its quantity of matter is greater. It 
makes no difference whether the boat is drawn to- 
ward the ship by a man standing in the boat and pull- 
ing at a rope fastened to the ship, or by a man stand- 
ing in the ship and pulling by a rope fastened to the 
boat. A fisherman once fancied he could manufacture 

a breeze for himself 
by mounting a pair 
of huge bellows in 
the stern of his boat, 
and directing the 
blast upon the sail. 
But he was surprised 
to find that it had no 
effect on the motion 
of the boat. We see 
that the reaction of 
the blast would tend 
to carry the boat 
backward just as much as its direct action tended to 
carry the boat forward. 

33. FALLING BODIES. When a body falls freely 
toward the earth from some point above it, it falls con- 
tinually faster and faster the longer it is in falling. Its 
motion therefore is said to be uniformly accelerated. 
All bodies, moreover, light and heavy, would fall 
equally fast were it not for the resistance of the air, 
which buoys up the lighter body more than it does the 

Compare the momentum of a small boat with that of a large ship when 

drawn together. Case of a man who put a pair of bellows to his boat. 

33. When is the motion of a body said to be uniformly accelera- 


heavier ; but in a space free from air, or a vacuum, 
a feather falls just as fast as a guinea. If a boy knocks 
a ball with a bat on smooth ice, it will move on uni- 
formly by the impulse it has received ; but if several 
other boys strike it successively the same way, its 
velocity is continually increased. Now gravity is a 
force which acts incessantly on falling bodies, and 
therefore constantly increases their speed. If I as- 
cend a high tower and let a ball fall from my hand 
to the ground, it will fall 16 T ^ feet in one second, 64 
in two seconds, and 257i in four seconds ; that is, a 
body will fall four times as far in two seconds as in 
one, and sixteen times as far in four seconds as in 
one. Now four is the square of two, and sixteen is 
the square of four ; so that the spaces described by a 
falling body are proportioned, not simply to the times 
of falling, but to the squares of the times ; so that a 
body falls in ten seconds not merely ten times as far 
as in one second, but the square of ten, or a hundred 
times as far. 

34. Hence, when bodies fall toward the earth from 
a great height, they finally acquire prodigious speed. 
A man falling from a balloon half a mile high, would 
reach the earth in about half a minute. We seldom 
see bodies falling from a great height perpendicularly 
to the earth ; but even in rolling down inclined planes, 
as a rock descending a steep mountain, or a rail car 
breaking loose from the summit of an inclined plane, 
we see strikingly exemplified the nature of accele- 
rated motion. A log descending by a long wooden 
trough down a steep hill, has been known to acquire 
momentum enough to cut in two a tree of considerable 

ted 1 How would a guinea and a feather fall in a vacuum *? Case of 
a ball knocked on ice. How much further will a body fall in two 
seconds than in one 1 How are the spaces of falling bodies pro- 
portioned to the times of falling 1 

34.- In what time would a man fall from a balloon hah" a mile 
high 1 Where do we see the rapid acceleration of falling bodies 


size, which it met on leaping from the trough. At a 
great distance from the earth, the force of gravity be- 
comes sensibly diminished, so that if we could ascend 
in a balloon four thousand miles above the earth, that 
is, twice as far from the center of the earth as it is 
from the center to the surface, the force of attraction 
would be only one fourth of what it is at the surface of 
the earth, and a body instead of falling 16 feet in a 
second would fall only 4 feet. At ten times the distance 
of the radius of the earth, the force of gravity would 
be only one hundredth part of what it is at the earth. 
This fact is expressed by saying, that the force of 
gravity is inversely as the square of the distance from 
the center of the earth, diminishing in the same propor- 
tion as the square of the distance increases. As the 
moon is about sixty times as far from the center of the 
earth as the surface of the earth is from the center, if 
a body were let fall to the earth from such a distance, 
(the force 'of gravity being the square of 60, or 3600 
times less than it is at the earth,) the body would be- 
gin to fall very slowly, moving the first second only 
the twentieth part of an inch. Were a body to fall 
toward the earth from the greatest possible distance, 
the velocity it would acquire would never exceed 
about 7 miles in a second ; and were it thrown up- 
ward with a velocity of 7 miles per second, it would 
never return. This, however, would imply a velocity 
equal to about twenty times the greatest speed of a 

35. When a body is thrown directly upward, its as- 
cent is retarded in the same manner as its descent is 
accelerated in falling ; and it will rise to the height 

exemplified 1 Case of a tree leaping from a trough. How is the 
force of gravity at great distances from the earth *? How, 4000 miles 
off? How, at the distance of the moon 1 State the law by which 
gravity decreases. What velocity would a body acquire by falling 
from the greatest possible distance 1 How far would it go it thrown 
upward with a velocity of 7 miles per second 1 



Fig. 10. 

from which it would have fallen in order to acquire the 
velocity with which it is thrown upward. 

36. VIBRATORY MOTION. Vibratory motion is that 
which is alternately backward and forward, like the 
motion of the pendulum of a clock. 
A pendulum performs its' vibrations 
in equal times, whether they are long^ 
or short. Thus, if we suspend two 
bullets by strings of exactly equal 
lengths, and make one vibrate over a 
small arc and the other over a large 
arc, they will keep pace with each 
other nearly as well as when their 
lengths of vibration are equal . Long 
pendulums vibrate slower than short 
ones, but not as much slower as the 
length is greater. A pendulum, to 
vibrate seconds, must be four times 
as long as to vibrate half seconds ; 
to vibrate once in ten seconds it 
must be a hundred times as long as 
to vibrate in one second, the com- 
parative slowness being proportional 
to the square of the length. The motion of a pendu- 
lum is caused by gravity. If we draw a pendulum out 
of its position when at rest, and then let it fall, it will 
descend again to the lowest point, but will not stop 
there, for the velocity which it acquires in falling will 
be sufficient, on account of its inertia, to carry it to the 
same height on the other side, (Art. 35,) whence it 
will return again and repeat the same process ; and 
thus, were it not for the resistance of the air, and the 

35. When a body is thrown upward, in what manner is it re- 
tarded 1 How high will it rise 1 

36. Define vibratory motion. How are the times of vibration of 
a pendulum 1 Example. How much longer is a pendulum that 
vibrates seconds, than one that vibrates half seconds 1 How much 



friction at the center of motion, the vibration would 
continue indefinitely. 

37. It is the equality in the vibrations of a pendu- 
lum, which is the foundation of its use in measuring 
time. Time may be measured by any thing which di- 
vides duration into equal portions, as the pulsations of 
the wrist, or the period occupied by a portion of sand 
in running from one vessel to another, as in the hour- 
glass ; but the pendulum can be made of such a length 
as to divide duration into seconds, an exact aliquot 
part of a day, and is therefore peculiarly useful for this 
purpose. Since, also, the pendulum which vibrates 
seconds at any given place, is always of the same in- 
variable length, it forms the best standard of measures 
by which all others used by society can be adjusted 
and verified. 

38. PROJECTILE MOTION, A body projected into 
the atmosphere, rises and falls in a curve line, as 
when a stone is thrown, or an arrow shot, or a can- 
non ball fired. The body itself is called a projectile* 
the curve it describes, the path of the projectile, and 
the horizontal distance between the points of ascent 
and descent, the range. When an arrow is shot, the 
impulse, if it were the only force concerned, would 
carry it forward uniformly in a straight line ; but the 
gravity continually bends its course toward the earth 
and makes it describe a curve. An arrow, (or any 
missile,) will have the greatest range when shot at an 
angle of 45 with the horizon ; and the range will 
be the same at any elevation above 45 as at the 
same number of degrees below 45. A cannon 

longer to vibrate in 10 seconds than in 1 1 What causes the motion 
of a pendulum 1 Why does it not vibrate forever 1 

37. On what property of the pendulum is its use for measuring time 1 
What other modes are there 1 Why is the pendulum better than other 
modes 1 On what principle does it become a standard of measures'] 

38. When is a body called a projectile 1 What is the curve de- 
scribed called 1 The horizontal distance 1 At what angle of eleva- 



ball shot at an elevation of 60 will fall at the 

same distance from the gun as when shot at an angle 

of 30. Thus, in the annexed diagram, a ship is 

Fig. 11> 

fired on from a fort, as she is attempting to pass it* 
The ball fired at an elevation of 45, is the only one 
that reaches the ship : the others fall short, and equally 
when aimed above and below 45. 

39. If a cannon ball were fired horizontally from the 
top of a tower, in the direction of P B, the range would 
depend on the strength 
of the charge. With B P 

an ordinary charge, it 
would descend in the 
curve P D ; with a 
stronger charge, it 
would move nearer to 
the horizontal line and 
descend in PE. We 
may conceive of the 
force being sufficient to 
carry the ball quite 
clear of the earth, and 
make it revolve around 
it in the circle. 

tion must an arrow be shot, to have the greatest range 1 At what 
two angles would the ranges be equal 1 
39. Explain Figure 12. 



SEC. 3. Of the Center of Gravity. 

40. The center of gravity of a body is a certain 
point about which all parts of the body balance each 
other, so that when that point is supported, the whole 
body is supported. If across a perpendicular support^ 

Fig. 13. 


as G, (Fig. 13,) I lay a wire 
having a ball at each end, B C, 
there is one point in the wire, 
and only one, upon which the 
balls will balance each other. This point is the 
center of gravity of all the matter contained in the 
wire and both balls. It is as much nearer the larger, 
B, as the weight of this exceeds that of C. When two 
boys balance one another at the ends of a rail, the 
lighter boy will require his part of the rail to be as 
much longer as his weight is less. The center of 
gravity of a regular solid, as a cube, or a sphere, lies 
in the center of the body, when the structure of the 
body is uniform throughout ; but when one side is 
heavier than another, the center of gravity lies toward 
the heavier side. 

41. The line of direction is a line drawn from the 
center of gravity of a body perpendicularly to the 
Fig. 14. Fig. 15. 

F F 

horizon. Thus, G F, (Fig. 14 or 15,) is the line of 
direction. When the line of direction falls within the 

40. Define the center of gravity. Explain Figure 13. 

41. Explain Figure 14. Where is the center Of gravity of a regu- 
lar figure fcjluciled '* 



Fig. 16. 

base, (as in Fig. 14,) or part of the body on which it 
rests, the body will stand ; when this line falls without 
the base, (as in Fig. 15,) the body will fall. At Pisa, 
in Italy, is a cele- 
brated tower, called 
the leaning tower. It 
stands firm, although 
it looks as though it 
would fall every mo- 
ment ; and being ve- 
ry high, a view from 
the top is very exci- 
ting. Yet there is no 
danger of its falling, 
because the line of 
direction is far with- 
in the ba$e. To ef- 
fect this, the lower 
part of the tower is 
made broader than 
the upper parts, and ^^gjj^ 
of heavier materials. 
These two precautions carry the center of gravity low. 
Structures in the form of a pyramid, as the Egyptian 
pyramids, have great firmness, because the line of di- 
rection passes so far within the base. 

42. If we stick a couple of pen- 
knives in a small bit of wood, and poise 
them on the finger, or adjust them 
so that the center of gravity will fall 
in the line of a perpendicular pin, the 
point of the wood will rest firmly on 
the head of the pin, so that the knives 
may be made to vibrate on it up and 
down, or to revolve around it, with- 

Fig. 17. 

Define the line of direction. Explain Fig. 16, Tower of Pisa. 
Why are pyramids so firm 1 



Fig. 18. 

out falling off. A loaded ship is not easily over- 
turned, because the center of gravity is so low, that 
the line of direction can hardly be made to fall with- 
out the base ; but a cart loaded with hay or bales of 
cotton is, on the other hand, easily upset, because 
the center of gravity is so high. A stage coach 
carrying passengers or baggage on the top, is much 
more liable to upset than it is when the load is all on 
a level with the wheels. A round ball, however 
large, will rest firmly on 
a very narrow base, be- 
cause the center of grav- 
ity (which is in the cen- 
ter of the ball) is always 
directly over the point of 
support ; and, according 
to the definition, when ' 
this is supported, the bo- 
dy is supported. In the 
annexed diagram, a hea- 
vy ball, connected with 
the figure, bends under 
the table, and thus brings 
the center of gravity of 
the whole within the base, 
so that the animal rests firmly on his hind legs. 

43. Animals with four legs walk sooner and more 
firmly than those with only two, because the line of 
direction is so much more easily kept within the 
base. Hence, children creep before they walk, and 
the art of walking, and even of standing firmly, re- 
quires so nice an adjustment of the center of gravity, 
(which must always be kept over the narrow base 

42. Explain Fig. 18. Why is not a loaded ship easily overturn- 
ed *? A cart loaded with hay a stage coach a round ball 1 

43. Why do four-legged animals walk sooner than two-legged 7 
Why do children creep before they walk 1 


within the feet,) that it is learned only after much ex- 
perience. Children at school, also, are sometimes di- 
rected to turn out their toes when they walk, and to 
extend one foot from the other in taking a position to 
speak, because such attitudes, allowing a broader 
base for the line of direction, appear more firm and 

44. A boy promised another a cent, if he would 
pick it up from the floor, standing with his heels close 
against the wall. But in attempting to pick it up, 
he pitched upon his face. Performances on the slack 
rope, which often exhibit astonishing dexterity, depend 
upon a skilful adjustment of the center of gravity. 
The process is sometimes aided by holding in the 
hand a short stick loaded with lead, which is so flour- 
ished on one side or the other, as always to keep the 
center of gravity over the narrow base. Among the 
ancients, elephants were sometimes trained to walk a 
tight rope ; a feat which was extremely difficult on 
account of the great weight of the animal. 

45. Bodies subject to no other forces than their 
mutual attraction, and in a situation 'to approach each 
other freely, will meet in their common center of 
gravity. If the earth and moon were left to obey fully 
their attraction for each other, they would immediate- 
ly begin to approach each other in a direct line, mov- 
ing slowly at first, but swifter and swifter, until they 
would meet in their common center of gravity, which 
would have its situation as much nearer to the earth 
as the weight of the earth is greater than that of 
the moon. So all the planets and the sun, i aban- 
doned to their- mutual attraction, would rush together 
to a common point, which on account of the vast 
quantity of matter in the sun, lies within that body. 

44. Case of picking up the cent. Performances on the slack rope 
by men, and even by elephants, explained. 

45. Where will bodies meet by their mutual attraction 1 Examples 


Indeed, were all the bodies in the universe abandoned 
to their mutual attraction, they would meet in their 
common center of gravity. 

SECTION 4. Of the Principles of Machinery. 

46. The elements of all machines are found among 
the Mechanical Powers, which are six in number the 
Lever, the Wheel and Axle, the Pulley, the Screw, 
the Inclined Plane, and the Wedge. That which 
gives motion is called the power ; that which receives 
it, the weight. The first inquiry is, what power, in 
the given case, is required just to balance the weight. 
Any increase of power beyond this, would of course 
put the weight in motion. It is a general principle in 
Machines, that the power balances the weight when it 
has just as much momentum. Now we may give a 
small power as much momentum as a great weight, by 
making it move over as much greater space in the 
same time, as its quantity of matter is less. One 
ounce may balance a thousand ounces, if the two be 
connected together in such a way that the smaller 
mass, when they are put in motion, moves a thousand 
times as fast as the larger. If the momentum of the 
power be increased beyond that of the weight, as may 
be done by increasing its quantity of matter, then it 
will overcome the weight and make it move with any 
required velocity. Whatever structure connects the 
power and the weight is a machine. 

47. THE LEVER. Figure 19 represents a lever of the 
simplest kind, where P is the power, W the weight, and 

in the moon and earth, and all the bodies of the solar system final- 
ly, all the bodies in the universe. 

46. What are the elements of all Machines 1 Enumerate the six Me- 
chanical powers. Distinguish between the power and the weight ? 
What is the first inquiry respecting the power 1 What is a general 
principle in Machines, respecting momentum ? How may we give 
a small power as much momentum as a great weight 1 How may 
one ounce balance a thousand 1 What happens when the momen- 
tum of the power is increased beyond that of the weight 1 What 
does any structure that connects the power and the weight become 1 


F the fulcrum, or point of support. Now P will just 
balance W when its weight 

is as much less as its dis- lg< ' 

tance from the fulcrum ispg |r 

greater. For example, if it 
is three times as far from 
the fulcrum as W, then one 

pound will balance three ; three pounds will balance 
nine ; and, universally, in an equilibrium, the power 
multiplied into its distance from the fulcrum, mil equal 
the weight multiplied into its distance. In the present 
case, where the longer arm of the lever is three times 
the length of the shorter, a power of ten pounds will 
balance a weight of thirty. 

Fig. 20. 

48. This principle is exemplified in a common pair 
of steel-yards. The same power is made to bal- 
ance different weights of merchandise by attaching 
W to the shorter and P to the longer arm, and placing 
P in a notch that is as much farther from the fulcrum 

47. Explain Fig. 19. State the general principle of the equilibri- 
um o.f the lever. Examples. 

48. Explain the principle of Steel-yards. How is the same 
power made to balance different weights 1 Explain the difference 



as its weight is less than that of the merchandise, 
W. Steel-yards have commonly a smaller and a 
larger side ; the former being ounce, and the latter 
quarter-pound notches. On examining such a pair of 
steel-yards, it will be seen that the hook to which the 
merchandise is attached, is four times as far from the 
fulcrum, when we weigh on the small, as when we 
weigh on the large side. Hence, we have to move 
the counterpoise over four notches on this side to gain 
as much povyer as we gain in one notch on the other. 
The spaces over which the power and weight move 
respectively, are in the same proportion. Thus, when 
the counterpoise is made to balance a weight ten 
times as large as itself, it will be seen, by making the 
arm of the steel-yards vibrate up and down, that the 
counterpoise moves ten times farther, in the same time, 
than the weight does, and of course with ten times the 
velocity. Hence the momenta of the power and the 
weight are the same. A crow-bar illustrates the same 
principle, when a man lifts a weight much heavier 
than the amount of force he applies, by making that 
force act at the longer end of the lever. A pair of 
shears is formed of two such levers combined; and 
the nearer we bring the article to be cut to the ful- 
crum, the greater is the mechanical advantage gained. 
Two boys differing in size, moving each other at the 
end of a pole laid across the fence, exemplify the same 

49. In the foregoing cases the weight and the pow- 
er are on opposite sides of the fulcrum, and it is called 
a lever of the first kind. When the power and 
weight are on the same side of the fulcrum, but the 
weight nearer to it than the povver, it is a lever of the 

between the smaller and the larger side. Show that the momenta 
of the counterpoise and weight are equal. Examples in a crow- 
bar a pair of shears boys on a rail. 
49. Distinguish between levers ofthe first, second, and third kinds. 


second kind, as in the fol- x Fig. 21. 

lowing figure. The m 
chariical advantage gain 
here is the same as in the L 
first, for the power moves 
as much faster than the 
weight as it is more dis- 
tant from the fulcrum. -w 
When the power and weight are both on the same 
side of the fulcrum, but the power nearer to it than 
the weight, it constitutes a lever of the third kind, as 

in figure 22. A door mo- 
> . ' i r IP 2 

ving on its hinges is a 

weight, the matter of F 
which, for our present 2, 
purpose, may be consider- 
ed as all collected in the ^ 
center of gravity, which, 

on account of the regular figure of the door, is the cen- 
ter of the door ; and the effects of any force applied to 
a body are the same as though all the matter was con- 
centrated in the center of gravity, and the force was ap- 
plied to that point. Now if, in shutting the door, I place 
my hand on the edge, this point being farther from the 
fulcrum than the center of gravity, I gain a mechan- 
ical advantage, because the power moves faster than 
the weight; but if I apply my hand nearer the ful- 
crum than the center of gravity, then the power moves 
slower than the weight, and operates under a mechan- 
ical disadvantage ; and as I approach nearer and 
nearer to the hinges, the door is shut with greater and 
greater difficulty. In the former case, the door exem- 
plifies the principle of a lever of the second kind ; in 
the latter, of the third. Suppose a ladder to lie on the 
ground, and it is required to raise it on one end by 

How may a door, in shutting, be either of the second or third kind! 
Example in a ladder. 



taking hold of one of the rounds. If I take hold of 
the lowest round, it will require a great effort to raise 
it, especially if the ladder is long. This effort will 
be less and less, until I come to the middle round, 
where I should neither gain nor lose any mechanical 
advantage, but should lift the ladder like any other 
body of the same weight, if raised directly from the 
ground by a string. If I apply my hand to any round 
beyond the middle, toward the farther end, I gain a 
mechanical advantage, and the greater as I approach 
nearer to the end of the ladder. We shall leave it to 
the ingenuity of the pupil to account for these several 

Fig. 23. 

t 50. THE WHEEL AND AXLE. The figure repre- 
sents a wheel, A N O, and axis, L M, where a small 
power w, balances a greater weight, W. The power 
required to balance the weight is as much less than 
the weight as the diameter of the axle is less than that 
of the wheel. The wheel and axle has a great analo- 
gy to the lever, and is indeed little more than a re- 
volving lever. For if the power were applied to the 

50. Explain Figure 23. How much less is the power than the 
weight 1 Show the analogy between the wheel and the lever. 
Explain Figure 24. 


end of one of the spokes of the wheel, that spoke, as it 
revolved, would describe the figure of a wheel. Thus, 

Fig. 24. 

the capstan of a ship is a large upright axle, having 
holes near the top into which long levers are inserted. 
The men press upon the ends of these and gain a me- 
chanical advantage in proportion as the length of the 
lever exceeds the radius of the axle. By this means 
they draw up heavy anchors. 

51. Wheels are much employed in machinery, and 
.serve very various purposes, although they do not al- 
ways act upon the principle of the wheel and axle, as 
just explained. In carriages, their chief use is to 
overcome friction, since a body that rolls on the 
ground meets with much less resistance than one that 
slides ; and in lifting a wheel over an obstacle, as a 
stone, a mechanical advantage is gained in the same 
proportion as the radius of the wheel exceeds that of 
the axle. Large wheels, therefore, overcome obstacles 
better than small ones. Wheels are much employed 
also to regulate velocity. Just step into a mechanic's 
shop and see this use exemplified in the turner's lathe. 
By passing a band over a large wheel that turns with a 
steady motion, one may convey that motion to the small 

51. What is the use of wheels in carriages'? What advantage is 
gained by rolling instead of sliding '"? Also, in overcoming obstacles 1 
Which gain most, large or small wheels 1 Use of wheels in regula- 



wheel of a lathe, and the smaller wheel will revolve 
as much faster than the larger as its diameter is less. 
Now by using small wheels of different diameters on 
the lathe, we may increase or diminish the velocity at 
pleasure. The same principle is illustrated in a com- 
mon spinning wheel, and in machinery for spinning 

52. In clock-work, there is usually a combination of 
a number of wheels, where one wheel is connected 
to the axis of another by a small wheel fastened to the 
called a pinion. Thus, the three wheels, A, B, 


Fig. 25. 

C, are connected. The power is applied to the whee 
A, on whose axis is the pinion a, the teeth of whicl 
or leaves, as they are called) catch into the teeth of 
~, whose pinion b in like manner turns the wheel C. 
Here the motion of each succeeding wheel is less 
than the preceding ; for if the pinion a have ten leaves, 
and the wheel B 100 teeth, the pinion in turning once 
would catch but ten teeth of the wheel, and must there^ 
fore turn ten times to turn B once. If the pinion has 

ting velocity. How exemplified in a turner's lathe 1 In a common 
spinning wheel 1 
52. Explain the use of wheels in clock-work. Explain Fig. 25. 



also 10 leaves, and the wheel C 100 teeth, then C 
turns ten times as slow as B and a hundred times as 
slow as A. By altering the proportions between the 
number of teeth in the wheel and leaves in the pinion, 
we may alter the velocity of a wheel at pleasure ; 
and this is the way in which wheels are made to move 
faster or slower, at any required rate, in clocks and 
watches. If we apply the power at the other end and 
let the wheel C act on the pinion b, and the wheel B 
on the pinion a, then B will turn ten times as fast as 
C, and A ten times as fast as B, and a hundred times 
as fast as C ; so that, when the wheels carry the pin- 
ions, the velocity is increased, but when the pinions 
.carry the wheels, it is diminished. 

53. THE PULLEY. A pulley is a grooved wheel, 
around which a rope is passed, and is either fixed or 
^novable. Figure 26 represents a fixed pulley ; and 
Fig. 26. Fig. 27. 


Show how the motion is accelerated in one direction and retarded in 

the other. How may we alter the velocity of a wheel at pleasure 1 

53. Define the Pulley. Name the two kinds. What is the use of 


Fig. 28. 

here no mechanical advantage is gained, since the 
power moves just as fast as the weight, and we must 
remember that it is only when the power moves faster 
than the weight, that any mechanical advantage is 
gained. The boy, however, in figure 27, draws 
himself up by lifting only half his weight, because the 
two ropes support equal portions of the weight. The 
principal use of the fixed pulley is to change the di- 
rection of the weight. Thus, in drawing a bucket out 
of a well, it is more convenient to pull downward by a 
rope passing over a pulley above the head, than upward 
by drawing directly at the bucket. By the movable 
pulley we gain a mechanical advantage, for by this we 
can give the weight a slower 
motion than the power has, and 
can proportionally increase the 
efficacy of the power. Thus, 
in figure 28, as both the ropes. 
A arid E, are shortened as the 
weight ascends, the rope to 
which P is attached is length- 
ened by both, and therefore P 
descends twice as fast as W 
rises, and the efficacy of the 
power is doubled. By employ- 
ing a pulley with a number of 
grooves (called a block) with 
a rope around each, we may 
make the power run off a great 
length of rope while the weight 
rises but little, being equal to 
the combined length by which all the ropes of the block 
are shortened. Thus, if the block carries twelve ropes, 
the power is increased in efficacy 12 times. Instead 
of a single block with a number of grooves, several 


the fixed pulley 1 Of the movable pulley 1 Explain the power of 
a block of pulleys. 


pulleys with single grooves are combined upon a simi- 
lar principle. By a block of pulleys, two men will 
lift a rock out of a quarry a thousand times as heavy 
as they could lift with their naked hands ; but the rope 
at which they pull will run off a thousand times as fast 
as the weight rises. 

54. THE INCLINED PLANE. The Inclined Plane be- 
comes a mechanical power in consequence of its sup- 
porting a part of the weight, and of course leaving only 
a part to be supported by the power. If a plank, for ex- 
ample, having on it a cannon ball, is laid flat on the 
ground, it supports the whole weight of the ball. If 
one end is gradually raised, more and more force must 
be applied to keep the ball from rolling down the 
plane : and when the plank becomes perpendicular, a 
force would be required to sustain the ball equal to its 
whole weight. We may therefore diminish the ef- 
fect of gravity, in ascending from one level to another, 
as much as we please, by making the inclination of the 
plane small. A builder who was erecting a large ed- 
ifice, had occasion at last to raise heavy masses of stone 
to the height of sixty feet. He might have hauled 
them up by pulleys ; but this was inconvenient, and be- 

54. How does the Inclined Plane become a mechanical power ? Ex- 



sides, pulleys are subject to so much friction as to oc- 
casion a great loss of power. He therefore con- 
structed of timbers and planks, an inclined plane six 
hundred feet long, and conveyed the blocks of stone 
up them on rollers. As the plane was ten times as 
long as it was high, it was as easy to roll 1000 pounds 
up the plane as it would have been to draw up 100 
pounds by a fixed pulley. But as the plane was ten 
times as long as it was high, the weight would have to 
pass over ten time's the space that it would if it had 
been raised perpendicularly by the pulley. In all 
cases, the mechanical advantage gained by the inclined 
plane is in the same proportion as its length exceeds 
its height. When a horse draws a loaded cart on lev- 
el ground, he has merely the friction to overcome ; but 
when he drags it up hill, he has, besides the friction, to 
lift a certain part of the load, which part will be great- 
er in proportion as the hill is steeper. If the rise is 
one part in ten, then he would lift one tenth of the load 

55. The SCREW. The screw is represented in the 
following diagram as acting 
upon a press, which is a very 
common use that is made of it. 
As the screw is turned, it ad- 
vances lengthwise through a 
space just equal to the dis- 
tance between the threads. 
Now if the power be applied 
directly to the head of the 
screw, then, in turning the 
screw once round, the power 
would move over as much 
more space than the screw advances, as the circum- 

ample. How employed in building 1 What makes it so hard to 
draw a load up hill q . 
55. Explain Fig. 30. How is the mechanical advantage gained, 

Fig. 30. 


Terence of the head is greater than the distance be- 
tween the threads. The mechanical advantage gained 
is in the same proportion ; and we may increase the 
efficacy of the power either by lessening the distance 
between the threads, or by increasing the space over 
which the power moves. If we attach a lever to the 
head of the screw, and apply the hand at the end, 
then we make the power move over a space vastly 
greater than that through which the screw advances, 
and the force becomes very powerful, and will urge 
down the press upon the books, or any thing in press, 
with great energy. 

56. THE WEDGE. The Wedge is an instrument 
used for separating bodies, or the parts of bodies, 
from each other, as is seen in the common wedge 
used for splitting rocks or logs of wood. In the kind 
of wedge in ordinary use, the mechanical advantage 
gained is greater in proportion as the wedge is thinner. 
Accordingly, it requires but a small force to drive a thin 
wedge, but a greater force in proportion as the thick- 
ness increases. Cutlery instruments, as knives, axes, 
and the like, act on the principle of the wedge. When 
long and proportionally thin, the wedge becomes a 
mechanical power of great force, sufficient to raise 
ships from their beds. 

57. MACHINES. Machines are compounded of the 
mechanical powers variously united. We recognise, 
at one time, the union of the lever with the screw ; at 
another, of the wheel and axle with the pulley ; and, 
at another, of nearly all the mechanical powers to- 
gether. The following figure represents a machine 
for hauling a vessel on the stocks, combining the 
wheel and axle, the screw, the inclined plane, and the 
pulley. Each contributes to increase the efficacy of the 

when the power is applied to the head of the screw! Also when 
applied at the end of the lever ? 

56. What is the Wedge used for 1 How is the mechanical advan- 
tage of i he \vedge incrca >d .' 



force, and all together make a powerful machine. A 

man applies his hand at B, and turns a crank which 

Fig. 31. 

acts on the principle of the lever upon the screw at D- 
If the space over which the hand moves in one revo- 
lution is a hundred times as great as the distance be- 
tween the threads of the screw, then the mechanical 
advantage gained is in the same proportion, and the 
force with which the screw urges the teeth of the 
wheel, is a hundred times that applied by the hand to 
the crank. The diameter of the wheel is four times 
that of the axle ; therefore, the force applied at E is 
four hundred times that at B. This acts on a com- 
bination of pulleys, which, having four ropes, multi- 
ply it again four times, and it becomes sixteen hun- 
dred. The inclined plane is twice as long as it is 
high, and therefore doubles the efficacy of the power, 
and it becomes three thousand and two hundred times 
what it was originally. So that the single force which 
a man can exert by means^of such a machine is pro- 
digious ; and if the machine was so contrived (as it 
might easily be) that a pair of horses or a yoke of 

57. How are Machines composed 1 PIxplain Fi. 31. How would 
the velocity of the weight compare with that of the power'? 


cattle, instead of the man, could turn the machine, the 
force would be adequate to move the largest ship. 
Such a machine, however, would move the body with 
extreme slowness. Its motion, in fact, would be dimin- 
ished as much as the efficacy of the power was in- 
creased. This, as we have said before, is a universal 
principle in mechanics ; so that we may find the power 
exerted by any machine, by seeing how much faster 
the moving force goes than the weight. 

58. Machines, therefore, gain no momentum: the 
power multiplied into its velocity always equals the 
weight multiplied into its velocity. But although 
machines do not of themselves generate any force, they 
enable us to apply it to much greater advantage to 
change its direction at pleasure to regulate its ve- 
locity and to bring in to the aid of the feeble powers 
of man the energies of the horse and the ox, of water, 
wind, and steam. 

59. FRICTION. The principles of machinery are first 
investigated, on the supposition that machines move 
without resistance from external causes. Then the 
separate influence of such accidental causes of irreg- 
ularity, in any given case, is ascertained and ap- 
plied. The two most general impediments to ma- 
chines are friction and resistance of the air, which 
occasion more or less destruction of force in all ma- 
chines. Friction arises from the resistance which 
different surfaces meet with in moving on each other. 
Perfectly smooth surfaces adhere together by a cer- 
tain force, opposing a corresponding resistance to the 
motion of the surfaces x on one another; but the as- 
perities which exist on most surfaces occasion a 
much greater resistance. An extreme case is when 

58. Do Machines gain any momentum 1 "What two products are 
always equal to each other 1 How do machines aid usl 

59. On what supposition are the principles of machinery first inves- 
tigated 1 What are the two general impediments to machines 1 




Fig. 32. 

one brush is slid across another, and the hairs inter- 
lace. By careful experiments on friction, the follow- 
ing are found to be its principal laws. First, the 
friction of a body, other things 
being equal, is proportioned to 
its weight. If a brick is laid 
on a table, with a string attach- 
ed to it connected with a scale 
below, by placing weights in 
the scale we may ascertain 
just how much force it takes 
to drag it off from the table 
under different circumstances, and this will be the 
measure of the friction. We should suppose that the 
friction would be greater on its broad than on its 
narrow side ; but experiments show that it is equal in 
the two cases, so that extent of surface makes no dif- 
ference when the weight remains the same. We 
may let the same brick rest on either side, and load 
it with different weights, equal to its own weight, 
double, triple, and so on. In all cases, we shall find 
the friction increased in the same proportion as the 
weight. Secondly, friction is increased by bodies 
remaining some time in contact with each other ; and 
when the contact is but momentary, as when a body 
is in very swift motion, the amount of friction is 
greatly diminished. Thus, when a carriage is in 
swift motion over a road, it encounters less resistance 
from friction in passing a given distance, than when 
it moves slowly. The same is strikingly the case in 
railway cars. 

60. Rolling are subject to far less friction than 
sliding bodies. Thus, if a coach wheel be locked, 
that is, made to slide down hill instead of rolling, its 

What causes friction 7 State an extreme case. To what is the fric- 
tion of a body proportioned ? How is the amount of friction affected 
by continued contact? 
60. Difference between rolling and sliding bodies 1 Use of lubrica- 


friction may be so much increased as to check the 
rapidity of "descent in any required degree. Rollers 
are therefore employed in transporting heavy bodies, 
to diminish friction ; and, for the same purpose, sur- 
faces are made smooth by applying grease, or different 
pastes, or even water, all of which fill up the inequali- 
ties and thus diminish the asperities of the surface. 
Although friction presents a resistance to machines, 
yet it has its uses in mechanical operations. It is 
this which makes the screw and the wedge keep their 
places ; and it is the friction of the surfaces of brick 
and stone against each other, which gives stability to 
buildings constructed of them. The wheels of a car- 
riage advance by their friction against the ground. 
On perfectly smooth ice they would turn without ad- 
vancing. We could not walk did not friction furnish 
us with a foothold ; and, it is for want of friction that 
walking is so difficult on smooth ice. So rail cars 
meet with great difficulty in proceeding when the rails 
have been recently rendered slippery by ice : the 
wheels turn without advancing. Friction is even em- 
ployed as a mechanical force, as when a lathe is 
turned by the friction of a band. Air meets with 
greater resistance in passing over rough surfaces than 
water does ; for water deposites a film of its own 
fluid upon the surface over which it moves, and thus 
lubricates it. Hence water flows in pipes with less 
resistance than air passes over the surfaces of a rough 
and sooty chimney. 

61. The resistance which bodies meet with in 
passing through air or water, increases rapidly as the 
velocity is increased, being proportioned to the square 
of the velocity. Thus, if a steamboat doubles its 

ting substances 1 Give examples of the uses of friction in the screw 
and the wedge in the materials of a building in carriage wheels 
in lathes. Which meets with the greater resistance from friction, 
water or air ? 
61. How is friction proportioned to velocity ? Example. 


speed, it encounters not merely twice as much resist- 
ance from the water, but four times as much. This 
makes it much more expensive to move boats rapidly 
than slowly, for it would require -nine times the force 
to triple the speed. 




62. HYDROSTATICS is that branch of Natural Phi- 
losophy, which treats of the pressure and motion of fluids 
in the form of water. 

SEC. 1. Of the PRESSURE of Fluids. 

63. Water, on account of the mobility of its parts, 
may be easily displaced, but it is with great difficulty 
compressed. If we take a hollow ball of even so 
compact a metal as gold, fill it full of w T ater, plug it 
close, and put it into a vise and compress it, the water 
will sooner force its way through the gold than yield 
to the pressure. This is an old experiment, and it led 
to the belief that water is wholly incompressible ; but 
it is now found that its volume may be reduced to 
smaller dimensions by subjecting it to very great pres- 
sures. Thus, 30,000 pounds pressure to the inch will 
lessen its bulk one twelfth. 

64. A fluid when at rest, presses equally in all direc- 
tions. A point in a tumbler of water, for example, 
taken at any depth, exerts and sustains the same 
pressure in all directions, upward, downward, and 
sidewise. So that if I attach a string to a musket 

62. Define Hydrostatics. 63. Is water compressible ? Experiment. 
What force is required to lessen its bulk one twelfth ? 
64. What is the law of pressure in all directions *? Example. 


ball and let it down into water, the weight of the water 
which rests on its upper side is balanced by an equal 
pressure on its under side. This is the most remark- 
able property of fluids, and is what distinguishes them 
from solids, which press only downward, or in the di- 
rection of gravity. 

65. A given pressure, or Now, impressed on any 
portion of a mass of water confined in a vessel, is dis- 
tributed equally through all parts of the mass. If I 
thrust a cork into a bottle filled with water, so near 
the top that the cork meets it, the pressure is felt, not 
merely in the direction of the cork, or just under it, 
but on all parts of the bottle alike ; and the bottle is 
as likely to break in one part as another, if equally 
strong throughout, and if not equally strong, it will 
give way at its weakest point, wherever that is situ- 
ated. If we insert into a large vessel of water a 
blown bladder, and then press upon 
the upper surface of the water with Fig. 33. 
a lid that fits it close, as in figure 33, 
the bladder will indicate an equal 
pressure on all sides. A is the lid 
that fits the jar, water-tight, and is 
applied to the top of the fluid ; B is 
a small blown bladder, kept in its 
place by a leaden weight resting on 
the bottom of the vessel. If a thin 
glass ball is substituted for the blad- 
der, on pressing down the lid, it will 
be broken into minute fragments, showing an equal 
pressure on all sides. The same effects would follow 
were the pressure applied at the side, or any other 
part of the vessel, instead of the top. 

66. This principle operates with astonishing power 
in the hydrostatic press. Figure 34 represents a press 

65. How is a pressure on any part of a confined mass of water 
distributed ? Example. Explain Figure 33. 



made of a strong frame of timbers, having a large 
cylinder, C D, full of water, 

Fig. 34. 


and opening into a small 
cylinder, A B, in which a 
plug (called a piston) is 
moved up and down by the 
lever attached to it. At 
D is another piston, which 
when forced upward press- 
es upon a follower at E, 
which communicates the 
force to a pile of books sup- 
posed in the process of bind- 
my hand to the lever and 
AB upon the surface of 
force it presses upon the 

ing. Now if I apply 
force down the piston in 
the water, with whatever 

surface of the fluid in the small cylinder, the same 
is exerted on all parts of the water in the large cylin- 
der, and consequently upon the piston D to push it 
upward against E. Suppose the number of square 
inches in the bottom of the piston E, is a thousand 
times as great as in that of the piston at B ; then by 
urging B forward with a force equal to one hundred 
pounds, I should communicate to E a pressure of one 
hundred thousand pounds. The water in the small 
cylinder would descend a thousand times as much as 
that in the large cylinder rose, so that the space 
through which the accumulated force could act would 
be very small ; still it would be sufficient for such 
articles as books, where the whole compression is but 
small. Since there is no loss from friction in this 
machine, a man can by means of it exert a greater 
power than by any other to which he can apply his 
own strength. He can by means of it crush rjocks, 

66. Describe the Hydrostatic Press. Suppose the number of 
square inches in the larger piston is a thousand times as great as 
in the smaller 1 Uses of the Hydrostatic Press 1 



and cut in two the largest bars of iron. The hydro- 
static press is much used as an oil press, as in ex- 
tracting oil from flaxseed ; and also for packing hay, 
cotton, and other light substances. 

67. The surface of a fluid at rest is horizontal. 
This property is applied to the construction of the 
FLUID LEVEL, used by carpenters, masons, and othe 


Fig. 35. 

workmen. It usually consists of a flat rule, having a 
horizontal glass tube on the upper side, containing 
alcohol, (which is preferred to water because it never 
freezes.) The tube is not quite full of the fluid, so 
that when laid on its side a bubble of air floats on the 
upper surface. When this is exactly at a given mark 
near the middle, then the surface on which the rule 
is laid is level. Figure -p. 36> 

36 represents a levelling 
staff much used in sur- * 
veying and grading 
lands. The liquid in the 
two arms of the tube at 
A and B being precisely 
on a level, any two re- 
mote objects, P and Q, 
may be brought accu- 
rately to the same level 
by sighting P with the 
eye at A ; that is, bring- 
ing it into the same hori- 
zontal line with the sur- ~~" 
faces of A and B, and 
then sighting Q in the same manner. 

67. How is the surface of a fluid when at rest 1 Describe the 
fluid-level and the levelling staff. 



68. The pressure upon any portion of a column of 
fluid, is proportioned to its depth below t\e surface. If 
we let down a junk bottle into the sea, the pressure 
on all sides of it would continually increase as it de- 
scended, until it would be sufficient to crush it. Its 
great strength, however, would enable it to bear a pro- 
digious pressure. When an empty bottle, corked 
closely, is let down to a great depth, on drawing it 
up, it is found full of salt water, and yet the cork un- 
disturbed. At a certain depth, the pressure on the 
cork is such as to contract its dimensions, and yet, 
being equally pressed on all sides, it is not displaced. 
Its size being contracted, the water runs in at the 
sides; but on rising to the surface, the cork swells 
again to its former bulk. When the stopper does not 
admit of compression, the water sometimes is forced 
through its pores, and thus fills the bottle. Ships sunk 
Fig, 37. a t 9- great depth, have their wood ren- 
\ dered so heavy by the great quantity of 
water forced into it, that when they go 
to pieces their parts do not rise. The 
pressure of water on a square foot, at the 
depth of eight feet, is 500 pounds ; and 
having the same amount added for every 
[ eight feet of descent, it soon becomes 
II prodigious. At the depth of a mile, it 
is no less than 330,000 pounds upon the 
square foot. 

69. Fluids rise to the same level in the 
opposite arms of a bent tube. Let Fig. 37 
be a bent tube : if water be poured into 
either arm of the tube, it will rise to the 
) same height in the other arm. Nor is it 
material what may be the shape, size, or 

68. How is the pressure of a column of fluid at different depths? 
Example in a junk-bottle. What happens to a corked bottle sunk 
to a great depth 1 What is the pressure on a square foot at the 
depth of eight feet and a mile ? 


inclination of the opposite arms. Figure 38 represents 
a variety of vessels and tubes open at top, but corn- 
Fig. 38. 

municating with a common cistern of water below. 
If we pour water into any one of these, so as to fill it 
to any height, the water will be at the same height 
in each of the others. Hence, water conveyed in 
aqueducts, or running in natural confined channels, will 
rise just as high as its source, and no higher. Be- 
tween the place of exit and the spring, the ground may 
rise into hills and descend into valleys, and the pipes 
which convey the water may follow all the irregulari- 
ties of the country, and still the water will run freely, 
provided no pipe is laid higher than the level of the 

70. The pressure of a column of water upon the lot- 
torn of a vessel, depends wholly upon the height of the 
column, without regard to its shape or size. In Fig. 
38 the pressure on the bottom of the cistern will be 
the same, whether one tube is attached, or the whole 
number, or the vessel itself is raised to the same 
height all the way of the same size as at the bottom, 

69. Fluids in the opposite arms of a bent tube ? What does Fig. 
38 represent 1 How high will water in an aqueduct rise 1 

70. Upon what does the pressure of a column of fluid on the bottom 



Fig. 39. 

or even if swelled out like a funnel, so as to be much 
larger above than below. On this principle is founded 
the hydrostatic paradox that any quantity of water 
however small may be made to raise any 
weight however great. Fig. 39 repre- 
sents a bellows having on one side 
an open tube communicating with it. 
On pouring water into the tube (the 
bellows being full) it will force up the 
top of the bellows, although loaded with 
heavy weights. A wine-glass of water, 
for example, will raise the boys that 
stand on the bellows, and would sensi- 
bly lift a weight many hundred time? 
as great. The principle is the same 
as in the hydrostatic press. Here the 
weight of the column of water affords 
the power that acts on the larger end of the bellows, 
as in the press the force of the piston in the small 
cylinder acts on that in the larger. 

SEC. 2. Of Specific Gravity. 

71. SPECIFIC GRAVITY is the weight of a body com- 
pared with another of the same bulk, taken as a stand- 
ard. Water is the standard for solids and liquids; 
common air for gases. The specific gravity of a 
mineral, for example, or of alcohol, is its weight com- 
pared with that of a mass of water of exactly the 
same volume ; the specific gravity of steam is its 
weight compared with that of the same volume of at- 
mospheric air. We must know, then, what an equal 
volume of the standard would weigh. This is ascer- 

of a vessel depend 1 State the principle called the hydrostatic para- 
dox. Explain Fig. 39. 

71. Define specific, gravity. What is the standard for solids what 
for liquids what for gases ? What must we knoiv in order to find the 



Fig 40. 

tained in the case of a solid, by finding how much 
less the body weighs in water than in air; and, in the 
case of a liquid or a gas, by weighing equal volumes 
of the body and of air. Wishing to know how much 
heavier a certain ore, which I suspected to be silver, 
was than water, I tried to compare its weight with 
that of an equal bulk of water ; but the ore being of 
very irregular shape, I found great difficulty in meas- 
uring it accurately to find the number of solid inches 
in it, so that I could weigh it against the same num- 
ber of inches of water. But learning that a body 
when weighed in water weighs as much less than 
when weighed in air, as is just equal to the weight 
of the same volume of water, I attached a string to 
the ore, hung it to one arm of the balance, and found 
its weight to be 4.75 ounces; 
and then bringing a tumbler of 
water under the suspended ore 
so as to immerse it, 1 found it 
did not in this situation weigh 
as much as before, but I had to 
take out 1.25 ounces to restore 
the balance. This, then, was 
what the ore lost in water, and 
was the weight of an equal 
volume of water. Now I have 
found that the ore weighs four 
ounces and three quarters, while 
the same bulk of water weighs 
only one ounce and a quar- 
ter. I see, therefore, at once, that the ore is about 
four times as heavy as water ; but to find the exact 
specific gravity, I see how many times the weight of 
the ore is greater than that of an equal volume of 
water, by dividing 4. "7 5 by 1.25, which gives 3.8 as 

specific gravity of a body 1 Describe the way of finding the speci- 
fic gravity of an ore also of" alcohol also of carbonic acid. 


the exact specific gravity of the ore. I conclude, 
therefore, that it cannot contain much silver, if any ; 
otherwise it. would be heavier. Again, desiring to 
find the specific gravity of some alcohol, (which is 
better in proportion as it is lighter,) I took a small 
vial, counterpoised it in a pair of delicate scales, 
and poured in water gradually till I had introduced 
exactly 1000 grains. I then set the vial on the 
table, and placing my eye accurately on a level with 
the surface of the water, I made a fine mark with a 
small file just round the water line. On emptying out 
the water and filling the vial to the same mark with 
the alcohol, I found the weight of it to be 815 grains. 
I therefore inferred that its specific gravity was 815, 
water being 1000. Having now my vial ready, I 
filled it to the mark successively with half a dozen 
different liquors, some lighter and some heavier than 
water, and thus found the exact specific gravity of each. 
Finally, I had the curiosity to see which is the heav- 
iest, common air, or that sort of air which sparkles so 
briskly in soda-water, and in bottled beer, called car- 
bonic acid. I therefore weighed a light glass bottle, 
which, as we commonly say, was empty, but was really 
filled with common air, and then withdrawing the air 
from the bottle by means of a kind of syringe which 
sucked it all out, I then turned the stop-cock attached 
to the mouth, shut the bottle close, and weighing it 
again, found it had lost 40 grains, which was the weight 
of the air. At last I filled the bottle with carbonic acid 
instead of air, and weighing again, found the vessel now 
weighed 60 grains more than before. This was the 
weight of the carbonic acid ; and now having found 
that when we take equal bulks of common air and 
carbonic acid, the latter weighs 60 grains, while the 
former weighs only 40, I infer that the carbonic acid 
is one half heavier than common air ; that is, its spe- 
cific gravity is 1.5. By a similar process, I found 


that hydrogen gas, one of the elements of water, is 
more than thirteen times as light as air, being the 
lightest of all known bodies. 

72. A body floats in water at any depth, when its 
specific gravity is just equal to that of water. The 
human system is a little heavier than water, and there- 
fore tends to sink in it ; but if we strike the water 
downward, its reaction will keep us up, acting as it 
does in a direction opposite to that of gravity. A very 
slight blow upon the water is sufficient to balance the 
downward tendency, and therefore swimming becomes 
an easy matter when skillfully practised. As we lose 
in water as much of our weight as the same bulk of 
water would weigh, and that is nearly the whole, it is 
only the slight excess of our weight which we have 
to sustain in swimming. Indeed, if we could keep 
our lungs constantly inflated, we should require no re- 
action to keep us up, but should float on the' surface. 
Dr. Franklin when a boy swam across a river by the 
aid of his kite, which supplied the upward force neces- 
sary to sustain him, instead of the reaction of the wa- 
ter. Fishes are nearly of the same specific gravity 
as the water in which they live. They are supplied 
with a small air-bladder, which they have the power 
of compressing and dilating. When they wish to sink 
they compress this bladder, and their specific gravity- 
is then greater than that of the water ; and they easi- 
ly rise again by suffering the bladder to dilate. Birds 
float in the atmosphere on similar principles. Being 
but little heavier, bulk for bulk, than the air, very 
slight blows with their wings create the reaction in 
an upward direction, which is necessary to sustain 
them ; stronger blows cause the reaction to overbalance 

72. When does a body float in water "? How is the body sup- 
ported in simmming 1 How did Dr. Franklin swim across a river 7 
How do fishes ascend and descend 1 How do birds fly 1 


the excess of their specific gravity over that of the air, 
and they rise with the difference. 

73. When a body floats on the surface of water, it 
displaces as much weight of water as is equal to its 
own weight. Thus, if I place a wooden block weigh- 
ing four ounces in a tumbler of water even full, just 
four ounces of the water will run over, as we may 
ascertain by collecting and weighing it. Upon this 
principle ships float on water. In proportion as we 
lade the ship, it sinks deeper and deeper, the weight 
of water displaced always being exactly equal to the 
weight of the ship and cargo. The actual weight of 
the ship and cargo may be easily ascertained on this 
principle ; for if we float the ship into a dock of known 
size, containing a given quantity of water, the weight 
of the ship and cargo may be determined from the rise 
of the water, in the dock. A boy wished to find the 
tonnage of his boat. He 
- 41 * therefore loaded it as heavy 

as it would swim, and then 
transferred it to a small box 
which he had made, and of 
which he knew the exact 
[dimensions. He then poured 
! into the box a pound of wa- 
ter at a time, and when it 
Jhad settled to a good level, 
he made a mark at the water line, arid adding one 
pound of water at a time, he thus 1 had marks at differ- 
ent heights, from one pound up to twenty. He found 
that four pounds of water were amply sufficient to 
float his boat, and when the boat was laid upon it, 
the water rose on the sides to the nineteenth mark. 
Consequently the boat had raised the water fifteen 

73. How much water does a floating body displace ? Example 
Method of finding the tonnage of a ship? How did the boy find the 
tonnage of his boat 1 


marks, and its weight was of course fifteen pounds ; 
for it weighed just as much as the water would have 
weighed which it would have taken to raise the level 
from the fourth to the nineteenth mark. 

SEC. 3. Of the MOTION of Fluids. 

74. That part of hydrostatics which treats of the 
mechanical properties and agencies of running water, 
is called Hydraulics, and machines carried by water, 
or used for raising it, Hydraulic machines. It em- 
braces what relates to water flowing in open channels, 
as rivers and canals ; or in pipes, as aqueducts ; or 
issuing from reservoirs in jets and fountains ; or falling, 
as in dams and cascades ; or oscillating in waves. 
A river or canal is water rolling down hill, and would 
be subject to the same law as other bodies descending 
inclined planes, were it not for the numerous impedi- 
ments which oppose the full operation of the law. 
Now a body rolling down an inclined plane has its 
motion constantly accelerated, like a body falling per- 
pendicularly, gaining the same speed in descending 
the plane that it would in falling through the perpen- 
dicular height of the plane. Hence when a body rolls 
down a long plane without obstruction, it soon acquires 
an immense velocity, as is seen in a rock rolling down 
a long hill. In the same manner, a body of water 
descending in a river constantly tends to run faster 
'and faster, and would soon acquire a most destructive 
I momentum, were it not retarded by numerous coun- 
jteracting causes, the chief of which are the friction of 
;the banks and bottom, and the resistance occasioned 
V its winding course, every turn opposing an impedi- 
ment of more or less force. By such a circuitous 
route two benefits are gained the rapidity of the 

74. Define Hydraulics. What subjects does it embrace. 1 Are rivers 
subject to the laws of falling bodies 1 What benefits arise from 


stream is checked, and its advantages are more widely 
distributed. A river flows faster in the channel, to- 
wards the middle, than near the banks, because it is 
less retarded by friction ; and during a freshet the 
rapidity is greatly increased, because since the waters 
that are piled on the original bed are subject to little 
friction, they exhibit something of the accelerated mo- 
tion of bodies rolling freely down inclined planes. A 
very slight fall is sufficient to give motion to water 
where the impediments are slight. The Croton Aque- 
duct, that waters the city of New York, falls but one 
foot in a mile. Three feet fall per mile makes a 
mountain torrent. Some rivers do not fall more than 
500 feet in 1000 miles, or a foot in two miles, and re- 
quire a number of days, or even weeks, to pass over 
this distance. 

75. The Aqueducts which the ancient Romans 
and Carthaginians built for watering their cities, 
were among the greatest of their works, some of 
which have remained until the present day. Large 
streams were conducted for many miles, sometimes 
not less than a hundred, in open canals, carried through 
mountains and led over deep valleys, on stupendous 
arches of masonry. Some have supposed that the 
ancients must have been unacquainted with the prin- 
ciple, that water flowing in pipes will rise as high as 
its source, since, had they known this, they might 
have conveyed water in pipes instead of such expen- 
sive structures ; these might have ascended and de- 
scended, following all the inequalities of the face of 
the country, provided they were in no part higher 
than the head or spring. It is found, however, that 
they were acquainted with the principle, but prefer- 

the circuitous routes of rivers 1 What part of a river flows fastest ? 
Why do rivers run so swift during a freshet 1 What fall per mile 
have the Croton Water Works 1 
75, What of the Aqueducts of the Romans and Carthaginians * 


red to construct their aqueducts of open channels rath- 
er than pipes. Suitable pipes, at that age, would 
have been very costly. They are apt also to become 
clogged ; and although they might have followed the in- 
equalities of hills and valleys, yet when they descend- 
ed and ascended fa,r from the general level, they would 
be obliged to encounter an enormous pressure, since 
in a column of water, the pressure on any part is proi< 
portioned to the depth below the surface of the water, 
increasing five hundred pounds to the square foot for 
every eight feet of descent. A pipe, therefore, fifty 
feet deep and full of water, would have to bear a pres- 
sure at the lower part of more than three thousand 
pounds to the square foot, and must be made propor- 
tionally strong, and would be apt to leak at the joints. 
Even at the present day, it is found more eligible to 
water cities by open aqueducts than by pipes, as is 
done in the new Croton Water Works for watering the 
city of New York. Here an artificial river of the 
purest water is conveyed from the county of Westches- 
ter, forty-one miles above the city, to a vast reservoir 
capable of holding 150,000,000 of gallons, where it has 
opportunity to deposit any sediment or impurities it 
may have taken up on its way, and to absorb air, which 
gives it life and briskness. From the reservoir it is 
distributed to all parts of the city in pipes, affording 
an ample supply for domestic uses, for watering and 
washing the streets, and for extinguishing fires. 

76. When a plug is removed from the top of one of 
the pipes of an aqueduct, the water spouts upward in 
a jet ; for, since water thus situated tends to rise as 
high as its source, it will spout to that height when 
unconfmed. At least it would ascend to that height 

Were the ancients acquainted with the principle that water as- 
cends to the level of its source 1 Describe the Croton Water Works. 
76. Why does water spout from a pipe of an aqueduct 1 How 
high will it spout ? 



were it not for the resistance of the air, which pre- 
vents its attaining that full height. It is on this prin- 
ciple that fountains are constructed. If we open a 
vent in the side of a water-pipe, so as to let the jet 
out obliquely, it will form the curve of a parabola ; 
and by letting out the jet through different orifices, 
the curves may be varied, and beautiful and pleasing 
figures exhibited, as is shown at the Park Fountain 
in the city of New York. 

77. In building tall or deep cisterns, we must re- 
member, that the pressure on any part of the cistern 
increases with the depth, and hence that the lower 
parts require to be made stronger and closer than the 
upper, else they will either burst in pieces or leak. 
A philosopher wishing to provide a constant supply 
of water near his house, constructed a large cistern 
six feet high, and contrived to convey a small stream 
of water to the top which kept it always full and run- 
ning over by a waste-pipe. In the side of the cistern 
he inserted two large stop-cocks of equal size, the 
first, one foot, and the other four feet from the top, 
supposing that he might, in a given time, draw off either 
one gallon or four gallons ; but he was surprised to find 
that he could obtain from the lower stop-cock only 
twice as much as from the upper. How, thought he, 
is this consistent with the principle that the pressure 
is proportioned to the depth ? If it presses against 
the side of the cistern at the lower level four times as 
much as at the upper, why do not four times as many 
gallons run out when the stopper is opened ? On re- 
flection, however, he perceived that the pressure on 
the side must be proportioned to the momentum, which 
depends on two things the quantity of matter and 
the velocity ; and of course that twice the quantity of 

77. How must we provide for the strength of a pipe at different 
heights'? Relate the story of the philosopher drawing water from a 
cistern. To what is the quantity of water discharged from a cistern at 


water flowing with twice the velocity, would have 
just four times the momentum. Hence he learned the 
grand principle, that in a column of water kept con- 
stantly full, the quantity discharged from any orifice 
in the side, is proportioned to the square root of the 
depth below the surface of the fluid. So that, to draw 
off twice as much, we must make the opening four 
times as deep, and to draw off three times as much, we 
must make it nine times as deep. 

78. The philosopher tried another experiment with 
his cistern. He turned off the run of water that sup- 
plied the cistern, and then opened the upper stop- 
cock, and found it took just five minutes to draw off 
the water to that depth. He then let in the run that 
supplied the cistern and kept it constantly full. Now 
opening the same orifice again, and drawing off for five 
minutes more, he found that he caught just twice as 
much water as before. From this he inferred, that if 
a vessel discharges a certain quantity of water in emp- 
tying itself to a certain level, it will discharge twice 
as much in the same time, when the vessel is keot 
constantly full. 

79. Water issues from the bottom or side of a ves- 
sel with the same force that it would acquire by fall- 
ing through the perpendicular height of the column. 
It would therefore seem to make no difference whether 
we let water fall upon a water-wheel from the top of 
a cistern, or whether we raise a gate at the bottom 
of the column, and let the water issue so as to strike 

the wheel there, since it would strike the wheel 
in both cases with the same velocity, except what 
might be lost in the falling column by the resistance 

different depths proportioned 1 How much lower must we go to 
double the quantity 1 

78. What other experiment did he try 1 How much more is dis- 
charged when the vessel is kept constantly full 1 

79. With what/orce does water issue from the bottom or side of a 
vessel 1 Does it make any difference whether water falls upon a 


of the air. A waterfall like that of Niagara, where 
an immense body of water rolls first in rapids down a 
long inclined plane, and then descends perpendicu- 
larly from a great height, affords one of the greatest 
exhibitions of mechanical power ever seen. The Falls 
of Niagara contain power enough to turn all the mills 
and machinery in the world. They waste a greater 
amount of power every minute, than was expended in 
building the pyramids of Egypt ; for, in that short 
space of time, millions of pounds of water go over the 
falls, and each pound, by the velocity it gains in fall- 
ing first down the rapids, and then perpendicularly, 
acquires resistless energy. Water falling one hundred 
feet would strike on every square foot with a force of 
more than six thousand pounds. 

80. Man imitates the power of the natural water- 
fall when he builds a dam across a stream, raising it 
above its natural level, and then turning aside more 
or less of it into a narrow channel, makes it acquire 
momentum while regaining its original level. When 
it has gained the requisite force, he turns it upon a 
water-wheel usually of great size, from which, by 
means of machinery, the force is distributed wherever 
it is wanted, and so applied as to do all sorts of work. 
When a run of water first strikes a wheel at rest, it 
strikes it with its full force ; but as the wheel moves 
before it, the effect of the force is diminished, and if 
the wheel acquired the same velocity as the stream, 
the force would become nothing. The wheel is re- 
tarded by making it do more and more work, or carry 
a greater weight, until it acquires a uniform motion at 
a certain rate, which ought to be that at which the force 
of the stream produces the greatest effect. This is 

wheel from the top, or issues upon it from the bottom 1 What of the 
Falls of Niagara 1 

80. When does man imitate the waterfall! With what force does a 
run of water ,/irs? strike a wheel 1 How when the wheel is in motion 1 



in some cases when the wheel moves half as fast as 
the stream. That a current of water or of wind strikes 
an object with less force when the object is moving the 
same way, is a general principle. Thus, when a steam- 
boat is moving directly before the wind, she would de- 
rive little aid from sails unless the wind were high, 
for she would " run away from the breeze ;" that is, 
the wind would produce no effect any farther than its 
velocity exceeded that of the boat, and if it were just 
equal to that, the effect would be absolutely nothing. 
A man in a balloon, carried forward by a wind blow- 
ing a hundred miles an hour, would speedily acquire 
the same velocity with the wind, and therefore appear 
to himself to be all the while in a calm. Although 
the earth is constantly revolving round the sun with 
inconceivable rapidity, yet as we have the same ve- 
locity we seem to be at rest. 

SEC. 4. Of the Remarkable Properties combined in 

81. Water combines in itself a variety of useful 
properties, all designed for the benefit of man. First, 
Natural History leads us to contemplate it in its va- 
rious aspects. It covers about three fourths of the 
globe, and is distributed into oceans, seas, and lakes, 
rivers, springs, and atmospheric vapor. By the agency 
of heat, water is constantly rising in vapor on all parts 
! of the ocean. This mingles with the air in an in- 
visible elastic state, being separated in the process of 
i evaporation from its salt and every other impurity. 
! More or less of it is conveyed over the land by winds, 
and falls upon it in dew, and rain, and snow. A part 
] of this filters through the sand, runs down in the 
i ==rr: 

, In what case does the stream produce its greatest effect 1 Example 
1 in a steamboat. How would a man in a balloon appear to hirn- 
\ self to be situated when moving with the same velocity as the wind 1 
81. What_parf of the earth is covered with water 1 In what dif- 
ferent forms '! What beneiits How from rivers ? Also from the ocean 1 


crevices of rocks, and collects in pure fountains not far 
below the surface, where it may be easily reached in 
almost every place, by digging wells. In various 
places it flows out by its own pressure, in springs and 
streamlets, which unite in rivulets, and these in 
rivers, which return the water to the sea. But rivers 
as they run are made to impart fertility, and to furnish 
an avenue by which vessels and steamboats may pene- 
trate into the heart of every country, and convey to the 
remotest cities the riches of every clime. As rivers 
furnish an entrance into the interior of countries, so 
the ocean forms the great highway between nations, 
and unites all nations in the bands of commerce. Still 
further, to serve the grand cause of benevolence, the 
ocean is filled with living beings innumerable, which 
are not, like land animals, confined to the surface, but 
occupy the depth of at least six hundred feet, and thus 
enjoy a far more extensive domain than the part of the 
animal creation that inherits the land. 

82. Secondly, Chemistry regards water with no 
less interest than Natural History. Its very composi- 
tion is admirable, being constituted of two substances, 
oxygen and hydrogen, which, when united with heat, 
are separated in the gaseous form, and each possesses 
the most curious and wonderful properties. Oxygen 
is found as an element in nearly all bodies in na- 
ture ; it is the part of atmospheric air which sus- 
tains all animal life and supports all fires ; and it is 
the most active agent in producing all the changes 
of matter which take place both in nature and art. 
Hydrogen gas is the most combustible of all bodies, 
and is in fact what we see burning in nearly every 
sort of flame. As a solvent, water performs the most 
useful service to man, removing every impurity from 
his clothing or his person, dissolving and prepar- 

82. What is the composition of water * What of oxygen and hydro- 
gen? What oi' water as a solvent ? Oi'the diiierem states oi' water 7 


ing his food, and entering largely into nearly all the 
processes of the arts. By the different states which 
water assumes, of ice and snow and vapor, it performs 
important offices in the economy of Nature, as well 
as in its native state of a liquid. These changes of 
state regulate the temperature of the atmosphere, and 
preserve it from dangerous excesses both of heat and 
cold. On the one hand, on the approach of winter in 

| cold climates, water changes to ice and gives out a 
vast amount of heat that kept it in the liquid state ; 
and on the approach of summer, to check the too 
rapid increase of temperature, the same heat which 
was given out when water was changed into ice, is 
now absorbed and withdrawn from the atmosphere, as 
ice is changed back to water. Moreover, during the 
heat of summer, the evaporation of water, a very 
cooling process, checks the tendency to excess of the 
heat of the sun, and guards us from all danger on that 
hand. Ice, by covering the rivers, keeps them from 
freezing except on the surface ; and snow is a warm 
and downy covering thrown over the earth to pro- 
tect the vegetable kingdom, by confining the heat of 
the earth. 

83. Thirdly, it is the province of Physiology to con- 
template the relations of water to the vegetable and 
animal kingdoms. Water is the chief food of plants, 
which it nourishes, either by supplying a part of their 
elements, or by dissolving their nutriment, and thus 
preparing it for circulation ; and hence water is indis- 

; pensable to the life and growth of all vegetables. To 
animals and man, it furnishes the best and only neces- 

jj sary beverage ; it is the medium by which our food is 

4 prepared ; and it acts medicinally in various ways, 

I both internally and externally. 

How does it check the cold of winter and the heat of summer 1 
Useful properties of ice and snow ? 

S3. What are the relations of water to the vegetable kingdom 1 
, What to animals and man ? 


84. Finally, the Mechanical relations of water, such 
as those we have been considering in the preceding 
pages, are hardly less remarkable and important than 
the rest. By its mobility, it maintains its own level 
and keeps itself within its prescribed bounds ; by its 
buoyancy, it furnishes a habitation for numerous tribes 
of fishes, and lays .the foundation of the whole art of 
navigation ; by its pressure in all directions, it gives the 
first indication of containing great mechanical energy, 
which is more fully developed in the immense force 
of running water, which may be regarded as a reposi- 
tory of power kept in readiness for the use of man ; 
and, finally, by its property of being converted into 
steam, it discloses a new and inexhaustible fountain of 
mechanical force, which man may employ in any 
degree of intensity to perform the humblest and the 
mightiest of his works. 




85. PNEUMATICS is that branch of Natural Philoso- 
phy which treats of the pressure and motion of elastic 
fluids. Elastic fluids are those which are capable of! 
contracting or dilating their volume under different 
degrees of pressure. They are of two kinds, gases 
and vapors. Gases constantly retain the elastic in- 
visjble state vapors remain in this state only when 
heated to a certain degree, but return to the liquid 

84. Advantages of its mobility of its pressure of its capacity of 
being converted into steam. 

;85. Define Pneumatics. What are elastic fluids *? State the two 
kinds and distinguish between them. What two elastic fluids are 


state when cooled. Common air is a gas, steam a 
vapor. Although there are many different gases and 
vapors known to Chemistry, yet air and steam are the 
elastic fluids chiefly regarded in Natural Philosophy. 
Air and steam are both commonly invisible ; but air, 
when we look through an extensive body of it, ap- 
pears of a delicate blue or azure color, which habit 
leads us to refer to distant objects seen through it. It 
is not the distant mountain that is blue, but the air 
through which we see it. Air also sometimes becomes 
visible when ascending and descending currents mix, 
as over a pan of coals, or a hot chimney, when we 
see a wavy appearance, which is air itself. Vapors 
also exhibit naturally some variety of colors, as yellow 
and purple ; but the vapor of water or steam is usually 
invisible. We must carefully distinguish between 
elastic vapor and the mist which issues from a tea- 
kettle. This is vapor condensed, or restored to the state 
of water, and it is only at the mouth of the tea-kettle, 
where it is hot, that it is in the state of steam, and there 
it is invisible. 

86. The general principles of mechanics apply to 
liquids and gases, as well as to solids, all bodies being 
subject alike to the laws of motion ; but the property 
of mobility of parts, which characterizes liquids, and 
of elasticity which characterizes gases and vapors, 
gives them severally additional properties, which lay 
the foundation of hydrostatics and pneumatics. Al- 
though we do not usually see gases and vapors, yet 
we find in them properties of matter enough to prove 
their materiality. In common with solids, they have 
impenetrability, inertia, and weight ; in common with 
liquids, they are subject to the law of equal pressure 
in all directions, and when confined they transmit the 

chiefly regarded in Natural Philosophy 1 When is air visible 1 Are 
vapors ever visible 1 

86. Do the general principles of Mechanics apply to liquids and 



effects of a pressure or blow upon any one part of th$ 
vessel, to all parts alike ; but in their elasticity, they 
differ from both solids and liquids. Since air and steam 
are the elastic fluids with which Natural Philosophy 
is chiefly concerned, we shall consider each of these 

SEC. 1. Of Atmospheric Air. 

87. We may readily verify upon atmospheric air, 
the various properties of an elastic fluid. Its impene- 
trability, or the property of excluding all other matter 
frorn the space it occupies, will be manifested if we 
invert a tall tumbler in water. It will permit the 
water to occupy more and more of the space as we 
depress it farther, but will never cease to exclude the 
water from a certain portion of the tumbler which it 
occupies. We may render this ex- 
periment more striking, by employ- 
ing a glass cylinder and piston, as is 
represented in Fig. 42. Let A B C D 
represent a hollow cylinder, made 
perfectly smooth and regular on the 
inside, and P a short solid cylinder, 
called a piston, moving up and down 
in it air-tight, and R the piston-rod. 
Now when we insert the piston near 
the top of the cylinder, the space 
below it is filled with air. On de- 
pressing the piston, the air, on ac- 
count of its elasticity, gives way, and we at first feel 
but little resistance ; but as we thrust it down nearer 
to the bottom, the resistance increases, and finally he- 
eases 1 What property characterizes liquids, and what solids 7 
W hat properties of matter have gases and vapors 1 

87. Show how air is proved to be material. Explain Figure 42. 
State the different principles which this apparatus is capable of 


omes so great that we cannot depress it any farther 
'by the strength of the hand. If we apply heavy 
weights, we may force it nearer and nearer to the 
bottom of the cylinder ; but no power will bring it into 
contact with the bottom. This experiment may be 
so varied as to prove several things. First, it shows 
that air is impenetrable ; secondly, that it may be 
indefinitely compressed all the air of a large room 
might be reduced to a thimble-full, and on removing 
the pressure, it would immediately recover its original 
volume ; thirdly, that the resistance increases the more 
it is compressed. We will graduate the cylinder into 
a thousand equal divisions, by horizontal marks num- 
bered from the bottom upward from one to one thou- 
sand, and place on the pan at the top of the piston-rod 
a few grains, so as just to overcome the friction of 
the piston against the sides of the cylinder. We will 
now put on weights successively, until we have sunk 
the piston half way, when the air occupies five hun- 
dred instead of a thousand parts of the cylinder. If 
we double the weight, it will not carry the piston the 
same distance as before, that is to the bottom, but only 
through half the remaining space, so that the air now 
occupies one fourth of the capacity of the cylinder. 
Jf we double the present weight, it will again be com- 
pressed one half, so as to fill but an eighth part of the 
cylinder. We find, therefore, that a double force 
of compression, always reduces to half the former 
volume. This law is expressed by saying, that the 
volume of a given weight of air is inversely as the com- 
pressing force. 

88. Air has the property of inertia. It remains at 

proving. How is the volume of a given weight of air proportioned 
to the compressing Ibrce 1 

88. -Why has air the property of inertia *? State the experiment 
which shows that air has weight. Why is air called a fluid 1 Have 
the particles of elastic fluids any cohesion 1 


rest unless put in motion by some force, and continues 
to move until some adequate force stops it.* When 
put in motion by any moving body, it destroys just as 
much motion in that body as it receives from it ; and 
it loses its motion only as it imparts the same amount 
to some other matter. A large body moving swiftly 
through the air meets with great resistance ; but what- 
ever motion it loses, it imparts to the air, which might 
be sufficient to produce a high wind. Air also has 
weight. If we balance a light bottle, containing a hun- 
dred cubic inches, in a delicate pair of scales, having 
just pumped out all the air from the bottle, and them 
open the stopper, and admit the air again, we find the 
vessel has gained in weight 30|- grains. We call air 
and all other gases and vapors fluids, because their 
particles move so easily among themselves. The par- 
ticles of elastic fluids have no cohesion, but on the other 
hand, have a mutual repulsion, which causes them to 
fly off from each other as soon as the compressing force 
is removed or diminished. 

89. The lower portions of air which lie next to the 
earth, are pressed by the whole weight of the atmo- 
sphere, which is found to amount to the enormous force 
of 15 pounds upon every square inch; or above 2,000 
pounds upon a square foot. This force would be insup- 
portable to man and animals, were it not equal in all 
directions, entering into the pores of bodies, and thus 
being everywhere nearly in a state of equilibrium. It 
is only when we withdraw the air from a given space, 
so as to leave the surrounding air unbalanced, that we 
see marks of this violent pressure. 

90. THE AIR-PUMP. Various properties of the air 
are exhibited by this beautiful and interesting appara- 
tus. A simple form of the Air-Pump is shown in fig- 

89. What is the pressure of the atmosphere upon a square inch, 
and square foot 1 Why is it not insupportable to man 1 



tire 43. A represents a cylinder having a piston 
moving up and down in it. The cylinder communi- 

Fig. 43. 

cates by an open pipe, B, with the plate of the pump, 
C, opening into the receiver, D, which is a glass ves- 
sel ground at the bottom so as to fit the plate of the 
pump air-tight. At S is a small screw which opens or 
closes a passage into the pipe, B, by which air may 
be let into the receiver when it has been withdrawn by 
the pump. 

In order to understand how the pump extracts the 
air from the receiver, or exhausts it, it is necessary 
first to learn the structure of a valve. A valve is any 
contrivance by which a fluid is permitted to flow one 
way, but prevented from flowing the opposite way. 
A common hand bellows affords an example of a valve, 
in the little clapper on the under side. When the 
bellows is opened, the clapper rises and the air runs 
in ; and when the bellows is shut, the clapper closes 

90. Air-Pump. Describe Fig. 43. Describe a valve. Example in 
a bellows in the piston and cylinder of the air-pump. Explain the 


upon the orifice, and as the air cannot escape by the 
same way it entered, it is forced out by the nozzle of 
the bellows. In the bottom of the cylinder, A, in fig- 
ure 43, there is a small hole, like a pin hole. On 
drawing up the piston, the space below it would be a 
vacuum were it not that the air instantly rushes in 
from the pipe, B, and the receiver, D, and fills the 
space, as water runs into a syringe. A strip of oiled 
silk is tied firmly over the orifice in the bottom of 
the cylinder on the inside, opening freely upward 
when this air seeks entrance from below, but shutting 
downward and preventing its return. Then if we 
should attempt to force down the cylinder, the air 
below it would resist its descent ; but a small hole is 
made through the piston itself, and a valve tied to the 
upper side opening upward ; so that on depressing 
the piston, the air below makes its way through the 
valve and escapes into the open space above. We 
raise the piston, and the air in the receiver follows it 
through a valve in the bottom of the cylinder opening 
upward. The original air of the receiver being now 
expanded equally through the receiver, the cylinder, 
and the connecting-pipe, we thrust down the piston, 
and the portion of the air that is contained in the cyl- 
inder is forced out through the piston. We again 
raise the piston, and the remaining air of the receiver 
expands itself as before through the vacuum ; we 
depress the piston, and a second cylinder full of air 
is withdrawn. By continuing this process, we rarefy 
more and more the air of the receiver, every stroke of 
the piston leaving what remains more rare than be- 
fore. Still, on account of the elasticity of air, what 
remains in the cylinder will always diffuse itself 
through the whole vessel, so that we cannot produce 
a complete vacuum by the air-pump. 

process of exhausting a vessel. Can we produce a complete va- 
cuum by the air-pump !? 


91. Several experiments will illustrate the great 
pressure of the atmosphere, when no longer balanced 
by an equal and opposite force. We shall find the 
receiver, when exhausted by the foregoing process, 
held firmly to the plate of the pump so that we cannot 
remove it until we have opened the screw, S, and 
admitted the air; then the downward force of the air 
being counterbalanced by an equal force from within, 
the vessel is easily taken off. The 
Magdeburg Hemispheres, represented in Fig. 44. 
figure 44, afford a striking illustration 
of the force of atmospheric pressure. 
When they have air within as well as , 
without, they are easily, when joined, , 
separated from each other ; but let us ' 
now put them closely together and 
screw the ball thus formed upon the 
plate of the pump, exhaust the air, and 
^close the stop-cock so as to prevent its 
return. We then unscrew the ball from the pump, and 
.screw on the loose handle ; the hemispheres are pressed 
so closely together that two men, taking hold by the 
opposite handles, can hardly pull them apart. Hemis- 
pheres four inches in diameter would be held togeth- 
er with a force equal to 188 pounds. Otto Guericke, 
of Magdeburg, in Germany, who invented the air-pump 
and contrived this experiment, had a pair of hemis- 
pheres constructed, so large that sixteen horses, eight 
on each side, were unable to dratv them apart. A 
pair only two feet in diameter, would require to sepa- 
rate them a force equal to 6785 pounds. If our bod- 
ies were not so penetrated by air, that the external 
pressure is counterbalanced by an equal force from 

91. Give an example of the great pressure of the atmosphere. De- 
scribe the Magdeburg Hemispheres. What is said of tt^se made 
by Otto Guericke 1 How much pressure does a middle-sized man 
sustain 1 Why are we not crushed 1 


within, we should be crushed under the weight of 
the atmosphere ; for a middle sized man would sus- 
tain a pressure of about 14 tons. 

92. If we take a square bottle, fit a stop- cock to it, 
and exhaust the air, the pressure on the outside will 
crush it into small fragments, with a loud explosion. 
It is prudent to throw a towel or handkerchief loose- 
ly over it, to prevent injury from the fragments. A 
square bottle is preferred to a round one, because 
such a figure has less power of resistance. The lap- 
stone experiment may be tried without an air-pump, 
and affords a pleasing illustration of the force of 
atmospheric pressure. Cut out a circular piece of 
sole leather, five or six inches in diameter. Through 
a hole in the center draw a waxed thread to serve as 
a handle. Soak the leather in water until it is 
very soft and pliable ; then, on applying this to any 
smooth, clean surface, as that of a lap-stone, a slab 
of marble, or a table, it will adhere with such force, 
that we cannot lift it off; but when we pull up- 
ward, the heavy body to which it is attached will 
be lifted with it. We may, however, slide it with 
ease, because no force acts up- 
Fig. 45. on it to prevent its motion in 

this direction, except simply 
the adhesion of the surfaces. 
Flies are said to ascend a pane 
of glass on this principle, by 
applying their broad feet firmly 
to the glass, which are held 
down by the pressure of the at- 
mosphere. When we apply a 
' sucker, and exhaust it with the 
mouth, the fluid rises because 

92. Describe the experiment with a square bottle. Also the lap- 
etone experiment. Why can we so easily slide the leather 1 How 
do flies ascend smooth planes 1 How does the boy suck water 1 



Fig. 46. 

it is forced up by the pressure of the atmosphere on its 
surface. When we draw in the breath, the lungs are 
expanded like a pair of bellows. Thus the air runs 
from the sucker into the lungs, and forms a vacuum in 
the sucker. Immediately the pressure of the atmo- 
sphere on the surface of the fluid, not being balanced 
in the tube, forces the fluid up the tube and thence into 
the mouth. 

93. If we fill a vial with water, and, placing one 
thumb on the mouth, invert it in a tumbler partly full 
of water, the water will not run out of 
the vial, but will remain suspended, 
because there being no air at the top of 
the column to balance the pressure that 
acts at the mouth of the vial, the 
column cannot descend. If, however, 
instead of the vial, we should employ a 
pipe more than 33 feet long, on filling it 
and inverting it, as was done with the 
Vial, the water would settle to about 33 
feet, and there it would rest ; for the 
pressure of the atmosphere is capable of 
sustaining a column of water only 33 
feet high. Were it higher than this, it 
would be more than a counterpoise for 
that pressure, and would overcome it 
and sink ; and were it lower than that, 
it would be overcome by that pressure, 
and rise until it exactly balanced the 
force of the atmosphere. Instead of fill- 
ing the pipe with water, we will attach a 
stop-cock to the open end, screw it on 
i the plate of the air-pump, and exhaust the air. We 
I will now close the stop-cock, and removing the tube 

93: Describe the experiment with the vial. Also with a pipe 
more than thirty-three feet long. If we exhaust the pipe arid open 
| it under water, what happens 1 


from the pump, will place the lower end of the pipe 
in a bowl of water. On opening the stop-cock the 
water will rush into the pipe, and rise to about the 
same height as before, namely, about 33 feet, where 
it will rest. In both cases, there is an empty space or 
vacuum in the upper part of the 'pipe above the column 
of water. 

Fig. 47. 94. This experiment illustrates the prin- 
ciple of the common pump, of the syphon, 
and of the barometer. Let us first <:see 
how water is raised by the pump. This 
"S apparatus usually consists of two pipes 
^=A a larger, A B, above., and a smaller, H C, 
~ below. The piston moves in the larger 
. pipe, and the smaller pipe descends into 
the well. On the top of the latter, where 
it enters the former, is a valve, V, opening 
upward. Suppose the piston, P, is down 
p close to this valve. On raising it, the air 
from the lower pipe diffuses itself into the 
r empty space below the piston, becomes 
H rarefied, and no longer balances the pres- 
sure of the atmosphere on the surface of 
B the well. Consequently, the water is 
forced up until the weight of the column, 
together with the weight of the rarefied 
air, restores the equilibrium. Suppose by 
the piston being drawn up to P, the water 
rises to H ; then the column, H C, and the 
rarefied air in both pipes together, first 
counterbalance the weight of the atmo- 
sphere. On raising the piston still higher, 
the water rises above H, but would not prob- 
ably reach the valve, V, by a single elevation of the pis- 


94. Explain the common pump from Fig. 47. How much force 
does the atmosphere exert in raising the water 1 


ton. We therefore thrust down the piston to repeat the 
operation. The air between V and P is prevented from 
returning into the lower pipe, by the valve, V, which 
shuts downward ; but the enclosed air, when com- 
pressed by the descending piston, lifts a valve in the 
piston, as in the air-pump, and escapes above. On- 
drawing up the piston a second time, suppose that the 
water rises into the upper pipe above the valve, V, then 
on depressing the piston again, this water, pressed on 
by the piston, lifts its valve, and gets above it. Finally, 
on drawing up the piston again, this same water is lifted 
up to the level of the spout, S, where it runs off. We 
exert just as much force in exhausting the air, as the 
pressure of the atmosphere exerts in raising the wa- 
ter. It requires, therefore, just as much force to raise 
a given quantity of water by the pump, as to draw it 
up in a bucket ; and the only question is, which is the 
most convenient mode of applying the force. 

95. The Syphon is a bent tube, 
having one leg longer than the 
other, as in Fig. 48. If we dip 
[the shorter leg into water and 
Isuck out the air from the tube, 
nhe water will rise, pass over the 
t;bend, flow out at the open end, 
;and continue to run until all the 
] water in the vessel is drawn off. 
IHere the pressure of the atmo- 
jsphere on both mouths of the tube 
iis the same ; but in each arm, that 
jpressure is resisted by the weight 
jof the column of water above it, and more by the 
jlonger than by the shorter column. This is the same 
Uhing as though the pressure were less upon the outer 

P5. Describe the Syphon. Why does it draw off the liquid'? State 
he uses of the Syphon. How high will it raise water 1 


than upon the inner mouth ; and it is easy to see that! 
if the water in a tube is pressed one way more thad 
the other, it will flow in the direction in which the- 
pressure is greatest. The syphon is used in dra wing- 
off liquors ; and the water in aqueducts is sometimes; 
conveyed over hills on the principle of the syphon., 
F . 4q But we must remember, that water couldj 
riot be raised by it more than 33 feet ; for] 
when the bend is 33 feet above the level of 
-si the fountain, then the column in the shorter 
arm balances the pressure of the atmosphere 
30 at the mouth of the tube in the well, and 
29 leaves no force to drive forward the column* 

into the descending arm. 

28 96. The Barometer is an instrument for 
27 measuring the pressure of the atmosphere. If 
the atmosphere be conceived to be divided^ 
into perpendicular columns, the barometer* 
measures the weight of one of these by the* 
height of a column of quicksilver which it 
takes to balance it. Quicksilver is 13-i- times| 
as heavy as water, and therefore a column so* 
much shorter than one of water, will balance] 
the weight of an atmospheric column. This 
will imply a column about 2i feet, or 3flj 
inches high ; and it will be much more conJ 
venient to experiment upon such a columnj 
than upon one of water 33 feet high. W6| 
will therefore take a glass tube about three* 
feet long, closed at one end and open at the* 
other, fill it with quicksilver, and placing the 
finger firmly on the open mouth, we will in- 
sert this below the surface of the fluid in the 
small cistern, as represented in the figure. 

96. Define the Barometer. Describe the mode of making it by 
Fig. 49. At what height will the quicksilver rest 1 What is the 
space above it called 1 


On withdrawing the finger, the quicksilver in the tube 
will settle to the height of about thirty inches, where 
it will rest, being sustained by the pressure of the 
atmosphere on the surface of the fluid in the cistern, 
to which force its weight is exactly equal. The space 
above the quicksilver, is the best vacuum we are able 
to form. It is called the Torricellian vacuum, from 
Torricelli, an Italian philosopher, who first formed it. 
The weight of a column of atmospheric air is different 
in different states of weather, and its variations will be 
indicated by the rising and falling of the quicksilver in 
the barometer. Any increase of weight in the air 
will make the fluid rise ; any diminution of weight 
will make it fall. Hence, these variations in the 
height of the barometric column, show us the compara- 
tive weight and pressure of the atmosphere at any 
given time. By applying to the upper part of the tube 
a scale divided into inches and tenths of an inch, we 
can read off the exact height of the quicksilver at any 
given time. Thus, the fluid, as represented in the 
figure, stands at 29.4 inches. 

97. The barometer is one of the most useful and 
instructive of philosophical instruments. By observing 
it from time to time, we may find how its changes are 
connected with the changes of weather, and thus it 
frequently enables us to foretell such changes. If, for 
example, we should observe a sudden and extraordinary 
fall of the barometer, we should know that a high 
wind was near, possibly a violent gale. To seafaring 
men, the barometer is a most valuable instrument, 
since it enables them to foresee the approach of a gale, 
and provide against it. As a general fact, the rising 
of the barometer indicates fair, and its falling, foul 

97. Explain the use of the barometer as a weather glass. "What 
would a sudden and extraordinary fall indicate 1 What weather 
does its rise, and what its fall indicate 1 



98. The foregoing considerations relate to the 
weight and pressure of the atmosphere ; but the air- 
pump also affords us interesting illustrations of the 
elasticity of air. We will fill a 
Fig. 50. vial with water, and invert it in 

a tumbler partly filled with the 
C~) same fluid. We will now place 

/*v >k the tumbler and vial on the plate 

of the air-pump, and cover it with 
a receiver, and exhaust the air. 
Soon after we begin to work the 
pump, we shall see minute bubbles 
of air making their appearance in 
the water, which will rise and col- 
^^_r~u-- -.~ ^ ect * n a bubble at the top of the 
column. The bubble thus formed, will expand more 
and more as the exhaustion proceeds, until it expels 
the water, and occupies the whole interior of the viaL 
This will happen much sooner if we let in a bubble of 
air at first, and do not wait for it to be extricated from 
the water ; but this extrication of air from the water, 
is itself an instructive part of the experiment, as it 
shows us that water contains a large quantity of air, 
held in combination with it by the pressure of the 
atmosphere on the surface, which pressure pervades 
all parts of the fluid alike. But on withdrawing this 
pressure gradually from the surface of the water, the 
particles of air imprisoned in the pores of the water 
escape, and collect on the top. The bubble thus 
formed, will expand more and more as the pressure is 
still farther removed, until it drives down the water 
and fills the whole vial. If we turn the screw S of 
the pump (Fig. 43) and let in the air, the pressure on 
the surface of the water in the tumbler being restored, 

98. Describe Fig. 50, and shovy how it illustrates the elasticity of 
air. What will porous bodies give out in an exhausted receiver 1 
How will warm water be affected 1 


the water will be forced up the vial again, and the 
air will be reduced to its original bubble. If we place 
any porous substance, as a piece of brick, or a crust 
of bread, in a tumbler, and fill the tumbler with water, 
(attaching a small weight to the bread to keep it under) 
we shall see, in like manner, an unexpected amount of 
air extricated when we place it under the receiver, and 
remove the atmospheric pressure from it, so as to per- 
mit it to assume the elastic state. Liquids boil at a 
much lower temperature than usual, when the pressure 
of the atmosphere is removed from them. Thus, if 
we take a tumbler half full of water, no more than 
blood- warm, set it under the receiver, and exhaust the 
air, it will boil violently. 

99. Air is the medium of combustion, of respiration) 
and of sound. If we place a lighted candle under the 
receiver of an air-pump, and exhaust the air, the light 
will immediately go out, showing that bodies cannot 
burn without the presence of air. Nor without this 
can animals breathe. A small bird placed beneath 
the receiver, will cease to breathe as soon as the air is 
exhausted. If a bell, also, is made-, to ring under a 
receiver, the sound will grow fainter and fainter as 
the air is withdrawn, and finally be scarcely heard at 
all. The buoyancy of air, like that of water, enables 
it to support light bodies. In a vacuum, the heaviest 
and lightest bodies descend to the earth with the same 
velocity. If we suspend a guinea and a feather from 
the top of a tall receiver, exhaust the air, and let them 
fall at the same instant, the feather will keep pace with 
the guinea, and reach the plate of the pump at the 
same instant. 

100. THE CONDENSER. A piston and cylinder may 
be so contrived as to pump air into a vessel instead of 

99. How may we show that air is essential to combustion'? Also 
to life'? Also to sound 1 Describe the guinea and feather experiment 



Fig. 51. 

pumping it out. Figure 51 represents a condensing 
syringe, screwed to a box partly filled with water. 
When the piston is drawn up to the top, above an ori- 
fice E in the side, the air runs in at 
E, which on depressing the piston, is 
driven forward into the box through 
a valve, V, which opens inward, but 
closes outward, and prevents the re- 
turn of the air. By repeated blows 
of the piston, more and more air is 
forced into the box, constantly in- 
creasing the pressure on the surface 
of the water. D is a tube opening 
and closing by a stop-cock, having its 
lower end in the water. When the 
air is strongly condensed, on opening 
the stop-cock, the water issues from 
the tube with violence. Soda Water 
Fountains are constructed on this prin- 
ciple. A great quantity of carbonic 
acid, or fixed air, is forced into a 
strong metallic vessel, containing a 
solution of soda, and therefore is sub- 
jected to a powerful pressure. A tube connects this 
vessel to the counter where the liquor is to be drawn, 
which issues with violence, as soon as vent is given to 
it, and foams, in consequence of the carbonic acid 
expanding by the removal of the pressure by which it 
had been confined. The condenser employed for this 
purpose, is called a forcing pump, and differs from the 
condensing syringe, represented in figure 51, chiefly in 
being worked by a lever attached to the piston, instead 
of the naked hand. 

100. Describe the Condenser from Fig. 51. How is air pumped 
into the box *? Explain the principle of soda water fountains. 

What is the forcing pump 1 


-Fig. 52. 


101. The Fire-Engine throws water by means of 
two forcing pumps, one on each side, which are work- 
ed by the firemen. T represents the hose, or leath- 
ern pipe, which leads off to some well or cistern of 
water, whence the supply is drawn. F is the work- 
ing beam, to each end of which is attached a piston 
moving in the cylinder A B. Suppose at the com- 
mencement of the process, the left hand piston is 
down close to the valve V ; as it rises, the water fol- 
lows it from the hose, lifting the valve V, and enter- 
ing P B below the piston. When the piston descends, 
it forces the water through a valve into the air- 
vessel, M. As the water is thrown in by successive 
descents of the piston, it rises in M, and condenses 
the air of the vessel into a small space at the top. A 
second hose, F, dips into the water, and terminates in 
the farther end in a pipe, which the fireman directs 

101. Describe the fire-engine from Fig. 52. 
used 1 Use of air-springs and air-beds 1 

Why is the air-vessel 


upon any required point, sending the water in a con- 
tinual stream. The stream might indeed be propel- 
led directly by the action of the pistons, without the 
intervention uf the compressed air in M ; but in that 
case it would go by jerks ; wHereas, the elasticity of 
the confined air acts as a uniform force, and makes 
the water flow out in a continual stream. Air-springs, 
acting on the same principle, are sometimes attached 
to coaches, and are said to operate well. Beds have 
been filled by inflating them with air instead of feath- 
ers, and have the advantage of being always made up. 

SEC. 2. Of Steam and its Properties. 

102. Steam, or the elastic fluid which is produced 
by heating water, owes its mechanical efficacy to its 
power of suddenly acquiring by heat a powerful elas- 
ticity, and then losing it as suddenly, by cold ; in 
the former case, expanding rapidly, and expelling 
every thing else from the space it occupies ; and, in 
the latter case, shrinking instantly to its original di- 
mensions in the state of water, and thus forming a va- 
cuum. By this means, an alternate motion is given 
to a piston, which being communicated to machinery, 
supplies a force capable of performing every sort of 
labor, and being easily endued with any required de- 
gree of energy, is at once the most efficient and the 
most manageable of all the forces of nature. Thus, 
if steam be admitted below the piston, in figure 53, 
when its force accumulates sufficiently to overcome 
the resistance of the piston, it raises it ; and if it then 
be let in above the piston, it depresses it. When the 
piston rises, it may be made to turn a crank half 
round, and the other half when it falls, and thus a 

102. To what two properties does steam owe its mechanical effi- 
cacy 1 To what is trie motion first communicated, and how trans- 
ferred to machinery 1 Show how the piston is raised and depressed. 


main wheel may be made to revolve, from which mo- 
tion may be conveyed to all sorts of machinery. The 
degree of force which steam exerts, depends on the 
temperature and density conjointly. If we put a 
spoonful of water into a convenient vessel, as an oil- 
flask, and place it over the fire, the water will soon 
be turned into elastic vapor, which will drive out the- 
air and fill the entire capacity of the vessel. As soon 
as this takes place, we cork the flask and again set it 
over the fire. The steam will increase in elastic 
power, just as that of air would do, which is only at 
a moderate rate, and it might be heated red hot with- 
out exerting any violent force. If we now unstop the 
flask and fill it one third full of water, and again place 
it on the fire, and stop it close when it is boiling free- 
ly, then successive portions of water will be constant- 
ly passing into vapor, and, of course, the steam in 
the upper part of the vessel will be constantly growing 
more and more dense. It is important to remember, 
therefore, that when steam is heated by itself, and 
not in contact with water, its elasticity increases 
slowly, and never becomes very great; but when it 
is heated in a close vessel containing water, which 
makes to it constant additions of vapor, thus increas- 
ing its density, it rapidly acquires elastic force, and 
the faster the longer the heat is continued, so as 
shortly to reach an energy which nothing can resist. 
! Such an accumulation of force sometimes takes place 
by accident in a steam boiler, and produces, as is 
well known, terrible explosions. 

103. If the foregoing principles are well under- 
i stood, it will be easy to learn the construction and 
l operation of the Steam Engine. For the sake of sim- 
plicity, we will leave out numerous appendages which 

Upon what does the degree of force depend 1 Experiment with a 
flask of steam with and without water. 



usually accompany this apparatus, but are not essen- 
tial to the main principle. In figure 53, A represents 

Fig. 53. 

the boiler, C the cylinder, in which the piston H moves, 
L the condenser, and M the air-pump. B is the steam- 
pipe, branching into two arms, communicating re- 
spectively with the top and bottom of the cylinder, and;! 
K is the eduction-pipe, formed of the two branches \ 
which proceed from the top and bottom of the cylin- 
der on the other side, and communicate between the 
pylinder and the condenser, which is immersed in a 
well or cistern of cold water. Each branch of the 
pipe has its own valve, as F, G, P, Q, which may be ' 
opened or closed as occasion requires. R is a safety 
valve, closed by a plate, which is held down by a 
weight attached to a lever, and sliding on it, so as 
to increase or diminish the force at pleasure. When 

103. Describe Figure 53. 


the force of the steam exceeds this, it will lift the 
valve and escape, thus preventing the danger of explo- 

104. Suppose, first, that all the valves are open, and 
that steam is issuing freely from the boiler. It is easy 
to see, that the steam would circulate freely through 
all parts of the engine, expelling the air, which would 
escape through the valve in the piston of the air-pump, 
and thus the interior spaces would be all filled with 
j steam. This process is called blowing off ; it is heard 
) when a steamboat is about leaving the wharf. Next 
*.the valves, F and Q, are closed, G and P remaining 
open. The steam now pressing on the cylinder, forces 
fit down, and the instant when it begins to descend, the 
istop-cock O is opened, through which cold water meets 
the steam as it rushes from the cylinder and condenses 
it, leaving no force below the piston to oppose its descent. 
Lastly, G and P being closed, F and Q are opened, the 
steam flows in from the boiler below the piston, and 
(rushes from above into the condenser, by which means 
jthe piston is forced up again with the same power as 
Jthat by which it descended. Meanwhile, the air-pump 
jis playing, and removing the water and air from the 
'condenser, and pouring the water into a reservoir, 
'whence it is conveyed to the boiler to renew the same 

105. In High Pressure engines, the steam is not 
Icondensed, but discharges itself directly into the atmo- 
sphere. The puffing heard in locomotives, arises from 
this cause. High pressure engines are those in which 
isteam of great density, and high elastic power, is used. 
,By this means, a more concentrated force is produced, 
jand the engine may be smaller and more compact ; 

104. Show how the engine is set a going, and kept at work. 
; 105. What becomes of the steam in high pressure engines 1 Whence 
arises the puffing heard in locomotives * What are high pressure 
engines 1 What are their advantages over low pressure engines 1 


but unless it is made proportionally stronger, it is more 
liable to explode, and when it gives way it explodes 
with great violence. 




106. METEOROLOGY is that branch of Natural Phi- 
losophy which treats of the Atmosphere. In Pneu- 
matics, we learn the properties of elastic fluids in 
general, on a small scale, and by experiment rather 
than by observation ; but in Meteorology, we extend 
our views to one of the great departments of nature, 
and we reason, from the known properties of air and , 
vapor, upon the phenomena and laws of the entire j 
body of the air, or the atmosphere. Meteorology leads! 
us to consider, first, the description of the atmosphere 1 
itself, including its extent, condition at different heights, 
and the several elements that compose it ; secondly, 
the relations of the atmosphere to water, including 
the manner in which vapor is raised into the atmo- 
sphere, the mode in which it exists there, and the 
various ways in which it is precipitated in the form 
of dew, fog, clouds, rain, snow, and hail ; thirdly, the 
relations of the atmosphere to heat, embracing the 
motions of the atmosphere as exhibited on a small 
scale, in artificial draughts and ventilation ; and on 
a large scale, in winds, hurricanes, and tornadoes ; 

106. Define Meteorology. How distinguished from Pneumatics 1 
What different subjects does Meteorology lead us to consider 1 


finally, in the relations of the atmosphere to fiery me- 
teors, as thunder and lightning, aurora borealis, and 
shooting stars. 

SEC. 1. Of the Extent, Density, and Temperature of 
the Atmosphere. 

107. The atmosphere is a thin transparent veil, 
enveloping the earth, and extending to an uncertain 
height, but probably not less than one hundred miles 
above it. Since air is elastic, and the lower portions 
next to the earth sustain the weight of the whole body 
of air above them, they are compressed by the load, as 
air would be under any other weight. As we ascend 
above the earth, the air grows thinner and thinner very 
fast, so that if we could rise to the height of seven 
miles in a balloon, we should find the air four times 
as rare there as at the surface of the earth. The air 
is, indeed, much more rare on the tops of high moun- 
tains than at the level of the sea ; and at a height 
much greater than that of the highest mountains on 
the globe, man could not breathe, nor birds fly. The 
upper regions of the atmosphere are also very cold. 
As we ascend high mountains, even in the torrid zone, 
the cold increases, until we finally reach a point where 
water freezes. This is called the term of congelation. 
At the equator, it is about three miles high ; but in 
the latitude of 40, it is less than two miles, and in the 
latitude of 80, it is only one hundred and twenty 
high. Above the term of congelation, the cold con- 
tinues to increase till it becomes exceedingly intense. 
The clouds generally float below the term of con- 
gelation. Mountains, when very high, are usually 
covered with snow all the year round, even in the 

107. Give a general description of the atmosphere, as to its height 
density at different herghts cold of the upper regions. What is the 
term of congelation 1 How high at the equator \ At 40 and fcO 7 


warmest countries, merely because they are above this 

SEC. 2. Of the Relations of the Atmosphere to 

108. Besides common air, the atmosphere always 
contains more or less watery vapor, a minute portion of 
fixed air, or carbonic acid, and various exhalations, 
which are generally too subtile to be collected in a 
separate state. By the heat of the sun, the waters on 
the surface of the earth are daily sending into the 
atmosphere vast quantities of watery vapor, which rises 
not only from seas and lakes, but even from the land, 
wherever there is any moisture. The vapor thus 
raised, either mixes with the air and remains invisible, , 
or it rises to the higher and colder regions, and isj 
condensed into clouds. Sometimes accidental causes 
operate to cool it near the surface of the earth, and! 
then it forms fogs. It returns to the earth in the forms! 
of dew, and rain, and snow, and hail. 

109. Dew does not fall from the sky, but is deposited' 
from the air on cold surfaces, just as the film of moisture^ 
is, which we observe on a tumbler of cold water in aJ 
sultry day. Here, the air coming in contact with! 
a surface colder than itself, has a portion of thel 
invisible vapor contained in it condensed into water.'.' 
In the same manner, on clear and still nights, which 
are peculiarly favorable to the formation of dew, 
the ground becomes colder than the air, and the 
latter circulating over it, deposits on it and on all 
things near it, a portion of its moisture. Dew does 
not form on all substances alike that are equally ex- 

108. What other elastic fluids besides air does the atmosphere con- 
tain 1 "Whence is the watery vapor derived 1 What becomes of it 1 

109. How is dew formed 1 Does dew form on all substances alike 1 
What receive the most 1 What receive none 1 


posed to it. Some substances on the surface of the 
earth are found to grow colder than others, and these 
receive the greatest deposit of dew. Deep water, as 
that of the ocean, does not grow at all colder in a 
single night, and therefore receives no dew ; and the 
naked skins of animals, being warmer than the air, 
receive none ; although the moisture which is con- 
stantly exhaled from the animal system itself, as soon 
as it comes into contact with the colder air that sur- 
rounds the person, may be condensed, and moisten the 
skin or the clothes in such a way as to give the appear, 
ance of dew. In this manner, also, frost (which is 
nothing more than frozen dew) collects, in cold weather, 
on the bodies of domestic animals. By a beautiful 
provision of Providence, dew is always guided with a 
frugal hand to those objects which are most benefited 
by it. Green vegetables receive much more than na- 
ked sand equally exposed, and none is squandered on 
the ocean. 

110. Rain is formed in the atmosphere at some 
distance above the earth, where warm air becomes 
cooled. If it is only cooled a few degrees, the moist- 
ure may merely be condensed into cloud ; but if the 
cooling is greater, rain may result ; and when a hot 
portion of air, containing, as such air does, a great 
quantity of watery vapor in the invisible state, is 
suddenly cooled by any cause, the rain is more abun- 
dant, or even violent. In such cases, it may have 
been cooled by meeting with a portion of colder air, 
as when a warm southwesterly wind meets a cold 
northwester, or by rising into the upper regions near 
the term of congelation. In some parts of the earth, 
as in Egypt, and in a part of Chili and Peru, it sel- 
dom or never rains, for there the winds usually blow 

110. Where is rain formed, and how 1 When is the precipitation 
in the form of cloud 1 When of rain 1 When is the rain violent * In 


steadily in one direction, and encounter none of those 
mixtures with colder air which form rain. In some 
other countries, as the northeastern part of South 
America, the rains are excessive ; and in others, as 
most tropical countries, the rains are periodical, being 
very copious at particular periods called the rainy 
seasons, while little or none falls during the other parts 
of the year. 

111. Snow is formed from vapor crystallized by 
cold instead of uniting in drops. By this means it is 
converted into a light downy substance, which falls 
gently upon the earth, and forms a covering that con- 
fines the heat of the earth, and furnishes an admirable 
defence of the vegetable kingdom, during winter, in se-i 
vere climates. In cold climates, flakes of snow consist 
of regular crystals, presenting many curious figures, 
which, when closely inspected, appear very beautiful. 
Nearly a hundred distinct forms of these crystals have 
been particularly described by voyagers in the polar! 
seas, specimens of which, as they appear under the] 
magnifier, are exhibited in the following diagram. 

When a body of hot air becomes suddenly and? 
intensely cooled, the watery vapor is frozen and 
forms hail. The most violent hailstorms are formed 
by whirlwinds, which carry up bodies of hot air far 
beyond the term of congelation, where the drops of 

what different ways is the hot air cooled ^ Where does it never rain 1 
Why 1 Where are the rains excessive 1 Where periodical 1 

111. Snow, how formed 1 What purpose does it serve 1 In what 
manner does it crystallize, and in how many different forms *? When 
is hail formed 1 How are the most violent hailstorms formed 1 How 


rain are frozen into hailstones, and these being sus- 
| tained for some time by the upward force of the whirl- 
I wind, accumulate occasionally to a very large size. 
Hailstorms are chiefly confined to the temperate zones, 
and seldom occur either in the torrid or the frigid 
zone. In the equatorial regions, the term of congela- 
tion is so high, that the hot air of the surface, if raised 
by a whirlwind, would seldom rise beyond it ; and in 
the polar regions, the air does not become so hot as is 
required to form a hailstorm. 

SEC. 3. Of the Relations of the Atmosphere to Heat. 

112. It is chiefly by the agency of heat, that air is 
put in motion. If a portion of air is heated more than 
the surrounding portions, it becomes lighter, rises, arid 
the surrounding air flows in to restore the equilibrium ; 
or if one part be cooled more than another, it contracts 
In volume, becomes heavier, and flows off on all sides 
until the equilibrium is restored. Thus the air is set 
in motion by every change of temperature ; and as 
such changes are constantly taking place, in greater 
or less degrees, the atmosphere is seldom at rest at 
any one place, and never throughout any great extent. 
The most familiar example we have of the effects of 
heat in setting air in motion, is in the draught of a 
chimney. When we kindle a fire in a fireplace, or 
stove, it rarefies the air of the chimney, and the denser 
air from without rushes in to supply the equilibrium, 
carrying the smoke along with it. Smoke, when 
cooled, is heavier than air, and tends to descend, and does 
descend unless borne up by a current of heated air. A 

do hailstones acquire so large a size * To what regions are hailstorms 
chielly confined 1 Why do they not occur in the torrid and frigid 

'! zones 1 

112. By what agent is air put in motion 1 Describe the process. 
How is the draught of a chimney caused 1 Why does smoke ascend 1 


hot current of air in a chimney is cooled much more 
rapidly when the materials of the chimney are damp 
than when they are dry, and therefore it will cool much 
faster in a wet than in a dry atmosphere. Hence, 
chimneys are apt to smoke in wet weather. It is es- 
sential to a good draught, that the inside of a chimney 
should be smooth, for air meets with great resistance 
in passing over rough surfaces. Burning a chimney 
improves the draught, principally by lessening the fric- 
tion occasioned by the soot. In stoves for burning an- 
thracite coal, it is important to the draught, that no air 
should get into the chimney except what goes through 
the fire. On account of the great resistance which a 
thick mass of anthracite opposes to air, this will not 
work its way through the coal if it can get into thej 
chimney by any easier route. Hence the pipes which 
conduct the heated air from a stove to the chimney, 
should be close, especially the joint where the pipe en- 
ters the chimney ; and care should be taken, that there 
should be no open fireplace, or other means of commu- 
nication, between the external air and the flue with 
which the stove is connected. 

113. It is important to health, that the apartments; 
of a dwelling-house should be well ventilated. This is? 
especially the case with crowded rooms, such as 
churches and schoolhouses. Of the method of venti-3 
lating churches, a beautiful specimen is afforded in the i 
Centre church, in New Haven. In the middle of the^ 
ceiling, over the body of the church, is an opening^ 
through the plastering, which presents to the eye nothing 
but a large circular ornament in stucco. Over this, 
in the garret of the building, a circular enclosure 
of wood is constructed, on the top of which is built a 

Why do chimneys smoke in wet weather 1 Why should a chimney 
be smooth 1 Why does burning a chimney improve the draught 1 
What precautions are necessary in burning anthracite coal, in order 
to secure a good draught 1 


large wooden chimney, leading off, at a small rise, to 
the end of the building, where it enters the steeple. 
An upper window of the steeple being open, in warm 
weather, the current sets upward from the church into 
the chimney? and thence into the tower, and completely 
i ventilates the apartment below. A door, so hung as 
to be easily raised or lowered by a string, leading to 
a convenient place at the entrance of the church, can 
be opened or closed at pleasure. In cold weather, it 
will generally be found expedient to keep it closed, 
to cut off cold air, opening it only occasionally. A 
schoolhouse may easily be ventilated by a similar con- 
trivance connected with a belfry over the center, as is 
done in several schoolhouses recently built in New 

114. Nature, however, produces movements of the 
atmosphere on a far grander scale, in the form of 
Winds. These are exhibited in the various forms of 
breezes, high winds, hurricanes, gales, and tornadoes ; 
varieties depending chiefly on the different velocities 
with which the wind blows. A velocity of twelve 
miles an hour makes a strong breeze ; sixty miles, a 
high wind, one hundred miles, a hurricane. In some 
extreme cases, the velocity has been estimated as high 
as three hundred miles an hour. The force of the 
wind is proportioned to the square of the velocity ; a 
speed ten times as great increases the force a hundred 
times. Hence, the power of violent gales is irresist- 
ible. Air, when set in motion, either on a small or 
on a great scale, has a strong tendency to a whirl- 
ing motion, and seldom moves forward in a straight 
line. The great gales of the ocean, and the smal. 

113. Ventilation, in what cases is it important 1 How effected 'n 
churches how in schoolhouses 1 

114. Specify the different varieties of winds. State the velocity. o r 
a breeze of a high wind of a hurricane. How is the Ibrce of a wind 
proportioned to tEe velocity 1 Tendency 01 air to a whirling motion 



tornadoes of the land, often, if not always, exhibit 
more or less of a rotary motion, and sometimes appear 
to spin like a top around a perpendicular axis, at the 
same time that they advance forward in some great 

cipal of these are the Thermometer, the Barometer, 
and the Rain Gage. The principle, construction, and 
uses of the Barometer, have already been pointed out, 
(Arts. 96 and 97.) Since it informs us of the changes 
that take place in the weight and pressure of the at- 
mosphere, at any given place, on which depend most 
of the changes of weather, it becomes of great aid in 
the study of Meteorology, and has, in fact, led to the 
knowledge of most of the laws of atmospheric phe- 
nomena hitherto established. We should, in pur- 
chasing, be careful to select an instrument of good 
workmanship, for no other is worthy of confidence. 
We should suspend it in some place where there is a 
free circulation of air as in an open hall, having an 
outside door and we should take the exact height of 
the mercury at the times directed below for recording 
the thermometrical observations. In case the barome- 
ter is falling or rising with unusual rapidity, observa- 
tions should be recorded every hour, or even oftener, 
as such observations afford valuable means of com- 
parison of the states of the atmosphere at different 

116. The Thermometer is an instrument used for 
measuring variations of temperature by its effects on 
the height of a column of fluid. As heat expands and 
cold contracts all bodies, the amount of expansion or 
contraction in any given case, is made a criterion of 

115. What are the three leading meteorological instruments ? 
Great value of the barometer. Rules for selecting a barometer and 
for observing. 

116. For what is the thermometer used 7 What shows the change 


1 the change of temperature. Fahrenheit's thermome- 
j ter, the one in common use, consists of a small glass 
1 tube, called the stem, with a bulb at one end, and a 
scale at the side. The bulb and a certain part of the 
stem are filled with mercury. The scale is divided 
into degrees and aliquot parts of a degree. If we dip 
the thermometer into boiling water, the mercury will 
expand and rise in the stem to a certain height, and 
there remain stationary. We will, therefore, mark 
that point on the stem, and then transfer the thermome- 
ter to a vessel where water is freezing. The mercury 
now descends to a certain level, and remains there sta- 
| tionary, as before. We mark this point, and we thus 
{ obtain the two most important fixed points on the scale, 
| namely, the freezing and boiling points of water. We 
I will now apply the scale, and transfer these marks from 
I the stem to the scale, and divide the part of the scale 
j between them into 180 equal parts, continuing the same 
I divisions below the freezing point 32 degrees, where 
1 we make the zero point, and there begin the graduation 
I from to 32, the freezing point, and so on 180 degrees 
j more, to 212, the boiling point. 

The best times for making and recording observa- 
I tions, are when the mercury is lowest, which occurs 
i] about sunrise, and when it is highest, which is near 
1 two o'clock in winter, and three in summer. The sum 
1 of these observations, divided by two, gives the aver- 
j age, or mean, for the twenty-four hours ; the sum of 
, the daily means for the days of a month, gives the mean 
1 for that month ; and the monthly averages, divided by 
I twelve, give the annual mean. By such observations, 
* any one may determine the temperature of the place 
I where he'^esides. 

] of temperature 1 Describe Fahrenheit's thermometer. How do we 
ascertain the boiling and freezing points of water 1 Into how many 
\ degrees is the space between them divided 1 Where is the zero 
i point, and at what degrees are the freezing and boiling points'! 
j How to find the daily, monthly, and annual means 1 



Fig. 55. 

117. The climate of the United States is very va- 
riable, and the annual range of the thermometer is 
greater than in most other countries. It embraces 
140, extending from 40 below zero, (usually marked 
40,) to 100 above. In the southern part of New 
England, the mercury seldom rises above 90, and de- 
scends but a few times in the winter below zero. From 
70 to 80 is a moderate summer heat. Although the 
equatorial regions of the earth are, in general, hotter 
than places either north or south, yet we have seen 
that the temperature of a place depends on various 
other circumstances, as well as on the latitude. (Arts. 
82 and 107.) 

118. The Rain Gage is an instrument 
employed for ascertaining the amount of 
water that falls from the sky, in the various 
forms of rain, snow, and hail. The sim- 
plest form is a tall tin cylinder, with a fun- 
nel-shaped top, having a graduated glass 
tube communicating with the bottom, and 
rising on the side. The water will stand 
, at the same level in the tube and in the cy- 
Minder, and the divisions of the tube may 
P be such as to indicate minute parts of an 
inch, and thus determine the depth of rain that falls on 
the area of the funnel, suppose a square foot. After 
the rain is over, the water may be removed by means 
of the stop-cock, and the apparatus will be ready for 
a new observation. It is useful to know the amount 
of rain that falls annually at any given place, not only 
in reference to a knowledge of the climate, but also 
for many practical purposes to which water is applied, 

117. What is said of the climate of the United States 1 What 
is the annual range of the thermometer 1 In New England, what is 
the range *? What is a moderate summer heat 1 

118. What is the Rain Gage *? Explain the simplest form. How 
to find the amount of rain fallen 1 Why is it useful to know the 
amount of rain that falls 1 


such as feeding canals, turning machinery, or irriga- 
ting land. 

SEC. 4. Of the Relations of the Atmosphere to Fiery 

119. The luminous phenomena which go under the 
general name of " fiery meteors," are Thunder Storms, 
Aurora Borealis, and Shooting Stars. Sudden and 
violent showers of rain, in hot weather, are usually 
accompanied by thunder and lig 'tning. The light- 
ning is owing to the sudden discharge of electricity, 
and the thunder is ascribed to the rushing together of 
the opposite portions of air, that are divided by the 
passage of the electric current. The snapping of a 
whip depends on the same principle as a, clap of thun- 
der. The lash divides the air, and the forcible meet- 
ing of the opposite parts to restore the equilibrium, 
produces the sound. Whenever hot vapor is rapidly 
condensed, a great amount of electricity is extricated. 
This accumulates in the cloud, until it acquires force 
enough to leap from that to some other cloud, or to the 
earth, or to some object near it, and thus the explo- 
sion takes place. 

120. The Aurora Borealis, or Northern Lights, are 
most remarkable in the polar regions, and are seldom 
or never seen in the torrid zone. They sometimes 
present merely the appearance of a twilight in the 
north; sometimes they shoot up in streamers, or ex- 
hibit a flickering light, called Merry Dancers ; some- 
times they span the sky with luminous arches, or 
bands ; and more rarely they form a circle with stream- 

119. What are the three varieties of fiery meteors ? How is 
lightning produced 1 To what is thunder ascribed 1 How explain- 
ed by the snapping of a whip 1 Origin of the electricity of thun- 

% <der storms 1 When does an explosion take place 1 

120. Aurora Borealis, where most remarkable 1 Specify the sev- 


ers radiating on all sides of it, a little southeast of tho 
zenith, called the corona. The aurora borealis is not 
equally prevalent in all ages, but has particular periods 
of visitation, after intervals of many years. It is 
more prevalent in the autumnal months than the other! 
parts of the year, and usually is most striking in the | 
earlier parts of the night, frequently kindling up with ] 
great splendor about 11 o'clock. From 1827 to 1842, 
inclusive, was a remarkable period of auroras. The \ 
cause of this phenomenon is not known ; it has been 
erroneously ascribed to electricity, or magnetism ; but | 
it is probably derived from matter found in the plan- 
etary spaces, with which the earth falls in while it is 
revolving around the sun. 

121. Shooting Stars are fire-balls which fall from 
the sky, appearing suddenly, moving with prodigious 
velocity, and as suddenly disappearing, sometimes 
leaving after them a long train of light. They are 
occasionally observed in great numbers, forming what 
are called Meteoric Showers. Two periods of the year 
are particularly remarkable for these displays, namely, 
the 9th or 10th of August, and the 13th or 14th of 
November. The most celebrated of these showers 
occurred on the morning of the 13th of November, 
1833, when meteors of various sizes and degrees ; 
of splendor, descended with such frequency as to 
give the impression that the stars were all falling 
from the firmament. The exhibition was nearly : 
equally brilliant in all parts of North America, and \ 
lasted from about 11 o'clock in the evening till sunrise. 
This phenomenon began to appear in some parts of 
the world, as early as November, 1830, and increased 

eral varieties. Is it equally prevalent in all ages 1 What was a re- 
markable period 1 Is its cause known "\ To what has it been as- 
cribed *? In what part of the year is it most frequent 1 

121. What are shooting stars 1 What two periods of the year are 
remarkable for their occurrence 1 When did the greatest meteoric 


I in splendor at the same period of the year, every year, 
; until 1833, when it reached its greatest height. It 
(was repeated on a smaller scale, every year, until 
1 1838, since which time nothing remarkable has been 
' observed at this period. The meteoric shower of 
August still (1843) continues. Meteoric showers ap- 
pear to rise from portions of a body resembling a 
comet, which revolves about the sun, and sometimes 
comes so near the earth that portions of it are attracted 
down to the earth, and are set on fire as they pass 
through the atmosphere. 




122. ACOUSTICS (a term derived from a Greek word 
which signifies to hear) is that branch of Natural Phi- 
losophy which treats of Sound. Sound is produced by 
the vibrations of the particles of a sounding body. 
These vibrations are communicated to the air, and 
by that to the ear, which is furnished with a curious 
apparatus specially adapted to receive them and con- 
vey them to the brain, and thus is excited the sen- 
sation of hearing. Vibration consists in a motion 
of the particles of a body, backward and forward, 
through an exceedingly minute space. The particles 
of air in contact with the body, receive a correspond- 
ing motion, each particle impels one before it, and re- 
shower occur 1 Describe this shower. Whence do meteoric show- 
ers arise 1 

122. Define Acoustics. How is sound produced 1 In what does 
vibration consist 1 Does it imply a progressive motion 1 What bo- 


bounds, and thus the motion is propagated from parti- 
cle to particle, from the sounding body to the ear. 
Such a vibratory motion of the medium, does not im- 
ply any current or progressive motion in the medium 
itself, but each particle recovers its original situation 
when the impulse that produced its vibration ceases. 
Elastic bodies being most susceptible of this vibratory 
motion, are those which are usually concerned in the 
production of sound. Such are thin pieces of board, 
as in the violin ; a steel spring, as in the Jewsharp ; a 
glass vessel, and cords closely stretched ; or a column 
of confined air, .as in wind instruments. If we stretch 
a fine string between two fixed points, and draw it out 
of a straight line to A, and then let it go, it will pro- 
ceed to nearly the same distance on the other side, to * 

E, whence it will return to B, and thus continue to 
vibrate through smaller and smaller spaces, until it 
comes to a state of rest. When we throw a stone 
upon a smooth surface of water, a circle is raised im- 
mediately around the stone ; that raises 'another circle 
next to it, and this another beyond it, and thus the 
original impulse is transmitted on every side. This 
example may give some idea of the manner in which 
sound is propagated through the air in all directions 
from the sounding body. 

dies are most susceptible of vibration 1 Give examples. Describe 
Fig. 56. What takes place Tvhen a stone is thrown on water 1 


123. Although air is the usual medium of sound, 
yet it is not the only medium. Solids and liquids, 
when they form a direct communication between the 
sounding body and the ear, conduct sound far better 
than air. When a tea-kettle is near boiling, if we 
apply one end of an iron poker to the kettle, and put 
the other end to the ear, we may perceive when the 
water begins to boil, long before it gives the usual signs. 
If we attacn a string to the head of a fire-shovel, and 
winding the ends around the fore fingers of both hands, 
apply them to the ears, and then ding the shovel 
against an andiron, or any similar object, a sound will 
be heard like that of a heavy bell. The ticking of a 
watch may be heard at the remote end of a long pole, 

jor beam, when the ear is applied to the other end ; 
; ; and if the watch is let down into water, its beats are 
.distinctly heard by an ear placed at the surface. A 
ibell struck beneath the water of a lake, has been 
ijheard at the distance of nine miles. Air is a better 
ijconductor of sound when moist than when dry. Thus, 
we hear a distant bell or a waterfall with unusual 
distinctness just before a rain, and better by night than 
by day. Air conducts sound better when condensed, 
land worse when rarefied. On the tops of some of the 
Ihigh mountains of the Alps, where the air is much 
irarefied, the sound of a pistol is like that of a pop-gun. 

124. The velocity of sound in air is 1130 feet in a 
.second, or a little more than a mile in five seconds. 
;On this principle, we may estimate the distance of a 
thunder-cloud, by the interval between the flash and 

ithe report. For example, an interval of five seconds, 
* gives 1130x5=5650 feet, or a little more than a mile. 
'A feeble sound moves just as fast as a loud one. Its 

123. Is air the only medium of sound *! Conducting power of 
solids and liquids 1 Experiment with a tea-kettle with a fire-shovel 
, with a watch. Conduct! no; power of moist air/? Of rarefied air 1 

124. Velocity of sound. How to estimate the distance of a thunder 



velocity is not altered by a high wind in a direction at 
right angles to the course of the wind ; but in the 
same direction, the comparatively small velocity of the 
wind is to be added, and in the opposite direction to be 
subtracted. In water, the velocity of sound is about 
four times as great as in air, being 4709 feet per 
second ; and in cast iron its velocity is more than ten, 
times as great as in air, being no less than 11,895 feet 
per second. 

125. Sound is capable of being reflected, and is thus 
sometimes returned to the ear, forming an eclw. Thus, 
the sound of the human voice is sometimes returned 
to the speaker, or other persons near him, in a repeti- 
tion usually somewhat feebler than the original sound ; 
but it may be louder than that, if several reflected \ 
waves are unitedly conveyed to the ear. When one ] 
stands in the centre of a hollow sphere or dome, j 
numerous waves being reflected from the concave 
surface so as to meet in the centre, a sound originally I 
feeble becomes so augmented as to be astounding. A I 
cannon discharged among hills or mountains, reverbe-| 
rates in consequence of the repeated reflexions of the! 

126. A sound becomes musical when the vibrations 
are performed with a certain degree of frequency.! 
The slow flapping of the wings of a domestic fowl hasj 
nothing musical ; but the rapid vibration of the wingsf 
of a humming-bird, produces a pleasant note. The 
slow falling of trees before a high wind, is attended 
with a disagreeable crash ; the rapid prostration of the 
trees of a forest by a tornado, with a sublime roar. 
A string stretched between two points, and made to 

cloud ? Velocity of a feeble sound effect of a high wind 1 Velocity 
of sound in water 1 

125. Echo, how produced when louder than the original sound 1 
Effect of a dome of a cannon among hills 1 

126. How a sound becomes musical V- examples in the wings of 
birds in falling trees in a vibrating string. How does increasing 


vibrate very slowly, has nothing musical ; but when 
the tension is increased, and the vibrations quickened, 
the note grows melodious. The strings of a violin 
give different sounds in consequence of affording vibra- 
tions more or less rapid. The larger strings, having 
slower vibrations, afford graver notes. The screws 
enable us to alter the degree of tension, and thus to 
increase or diminish the number of vibrations at plea- 
sure ; and by applying the fingers to the strings, we 
can shorten them more or less, producing sounds more 
or less acute, by increasing the number of vibrations 
in a given time. In wind instruments, as the flute, the 
vibrating body which produces the musical tone is the 
column of air included within. This, by the impulse 
given by the mouth, is made to vibrate with the requisite 
frequency, which is varied by opening or closing the 
stops with the fingers. The shorter the column, the 
more rapid is the vibration, and the more acute the 
sound ; and the length of the vibrating column is 
determined by the place of the stop that is opened, the 
higher stops giving sharper sounds because the vibrating 
columns are shorter. The pipes of an organ sound on 
a similar principle, the wind being supplied by a bellows 
instead of the breath. In certain instruments, as the 
clarinet and hautboy, the vibrations are first commu- 
nicated from the lips of the performer to a reed, and 
from that to the column of air. 

127. Sounds differing from each other by certain 
intervals, constitute musical notes. The singing of 
birds affords sweet sounds but no music, being uttered 
continuously and not at intervals. Man only, among 
animals, has the power of uttering sounds in this man- 

the tension, the size, or the length of the string, affect the pitch 1 
Example in the violin. What produces the musical tone in wind 
instruments'? Why does opening or closing the stops, alter the 
pitch 1 Explain the use of a reed. 

127. What sounds constitute musical notes'? Why is not the sing- 
ing of birds music 1 Why is man alone capable of uttering musical 


ner ; and his voice alone, therefore, is endued with 
the power of music. Music becomes a branch of 
mathematical science, in consequence of the relation 
between musical notes, and the number of vibrations 
that produce them respectively. Although we cannot 
say that one sound is larger than another, yet we can 
say that the vibrations necessary to produce one sound 
are twice or thrice, or any number of times, more 
frequent than those of another ; and the number of 
vibrations necessary to produce one note has a fixed 
ratio to the number which, produces another note. 
Thus, if we dimmish the length of a musical string one 
half, we double the number of vibrations in a given 
time, and it gives a sound eight notes higher in the 
scale than that given by the whole string, and is called 
an octave. Hence, these sounds are said to be to each 
other in the ratio of 2 to 1, because this is the ratio of 
the numbers of vibrations which produce them. A 
succession of single musical sounds constitutes melody ; 
the combination of such sounds, at proper intervals, 
forms chords ; and a succession of chords, produces 
harmony. Two notes formed by an equal number of 
vibrations in a given time, and of course giving the 
same sound, are said to be in unison. The relation 
between a note and its octave is, next after that of the 
unison, the most perfect in nature ; and when the two 
notes are sounded at the same time, they almost entirely 
unite. Chords are produced by frequent coincidences 
of vibration, while in discords such coincidences are 
more rare. Thus, in the unison, the vibrations are 
exactly coincident ; in the octave, the two coincide 
at the end of every vibration of the longer string, 
the shorter meanwhile performing just two vibra- 
tions ; but in the second, the vibrations of the two 

sounds 1 How does music become a branch of mathematical 
science 1 Example in a musical string. Define melody, chords, 
harmony, unison. How are chords produced 1 How discords 1 


I strings coincide only after eight of one string and nine 
of the other, and the result is a harsh discord. 

128. When an impulse is given to air contained in 
an open tube, the vibrations coalesce, and are propa- 
; gated farther than when similar impulses are made 
on the open air. Hence the increase of sound effect- 
ed by horns and trumpets, and especially by the speak- 
ing trumpet. Alexander the Great is said to have 
had a horn, by means of which he could give .orders 
to his whole army at once. Acoustic Tubes are em- 
ployed for communicating between different parts of 
a large establishment, as a hotel, or manufactory, by 
the aid of which, whatever is spoken at one extremity is 
heard distinctly at the other, however remote. They 
are usually made of tin, being trumpet-shaped at each 
end. They act on the same principle as the speaking 
trumpet. The Stethoscope is an instrument used by 
physicians, to detect and examine diseases of the lungs 
and the heart. It consists of a small pipe of wood or 
ivory with funnel-shaped mouths, one of which is ap- 
plied firmly to the part affected and the other to the 
oar. By this means the processes that are going on 
in the organs of respiration, and in the large blood- 
vessels about the heart, may be distinctly heard. 

128. Explain the effect of horns and trumpets. Use of Acoustic 
Tubes. How made 1 Explain the construction and use of the 
Stethoscope. / 






129. MORE than two thousand years ago, Theo- 
phrastus, a Greek naturalist, wrote of a substance we 
call amber, which, when rubbed, has the property of 
attracting light bodies. The Greek name of amber 
was electron, (TjXsx-r^ov,) whence the science was de* 
nominated ELECTRICITY. The inconsiderable expert 
ment mentioned by Theophrastus, was nearly all that 
the ancients knew of this mysterious agent ; but for 
two or tnree centuries past, new properties have been 
successively discovered, and new modes of accumu- 
lating it devised, until it has become one of the most 
important and interesting departments of natural sci, 
ence. It is common to call this power, whatever it 
is, the electric fluid, although it is of too subtile a 
nature for us to show it, as we do air, and prove that 
it possesses the properties of ordinary matter. But as 
it is more like an elastic fluid of extreme rarity, than 
like any thing else we are acquainted with, it is con- 
venient to denominate it a fluid, although we know very 
little of its nature. 

130. Some bodies permit the electric fluid to pass 
freely through them, and are hence called conductors / 
others hardly permit it to pass through them at all, and 

* The experiments in this chapter are so simple, and require so little appa-^ 
ratus, that it is hoped the learner will generally have the advantage of wit- 
nessing them, which will add much more than mere description to his im- 
provement and gratification. 

129. Explain the name electricity. What did the ancients know 
of this science 1 Its progress within two hundred years 1 Why is. 
electricity called a fluid 1 

130. Define conductors and non-conductors. Give examples of 


are therefore called non-conductors. Metals are the 
; Ibest conductors ; next, water and all moist substances ; 
land next, the bodies of animals. Glass, resinous sub- 
ttances, as amber, varnish, and sealing wax ; air, silk, 
wool, cotton, hair, and feathers, are non-conductors. 
Wood, stones, and earth, hold an intermediate place : 
they are bad conductors when dry, but much better 
when moist ; and air itself has its non-conducting 
power greatly impaired by the presence of moisture. 
Electricity is excited by friction. If I rub the side of 
;a dry glass tumbler, or a lamp chimney, on my coat 
sleeve, the electricity excited will manifest itself by 
i attracting such light substances as bits of paper, cot- 
jton, or down. A stick of sealing-wax, when rubbed, 
iexhibits similar effects. When an electrified body is 
^supported by non-conductors so that its electricity can- 
:j*iot escape, it is said to be insulated. Thus, a lock of 
(cotton suspended by a silk thread is insulated, because 
jif electricity be imparted to the cotton, it remains, 
' : since it cannot make its escape either through the 
.'thread, or through the air, both being non-conductors. 
|,A brass ball supported by a pillar of glass is insulated ; 
but when supported on a pillar of iron or any other 
i, metal, it is uninsulated, since the electricity does not 
remain in the ball, but readily makes its escape through 
the metallic support. By knowing how to avail our- 
selves of the conducting properties of some substances, 
and the non-conducting properties of other substances, 
we can either confine, or convey off the electric fluid 
at pleasure. 

131. There are a number of different classes of 
.phenomena which electricity exhibits ; as attraction 
and repulsion 4ieat and light shocks of the animal 
.system and mechanical violence. These will suc- 

each. How is conducting power affected by moisture 1 How is 
electricity excited 1 When is a body insulated 1 Give examples 
,of insulation. 


cessively claim our attention ; but as the properties of 
electricity were first discovered by experiment, so it 
is by experiments, chiefly, that they are still to be 
learned. We will therefore describe, first, a few such 
experiments as every one may perform for himself, 
and afterwards such as require the aid of an electrical 

SEC. 1. Of Electrical Attractions and Repulsions. 

132. For a few simple experiments, we will stretch a 
wire horizontally between the opposite walls of a room, 
or between any two convenient points, as represented 
in figure 57. This will afford a convenient support 

Fig. 57. 



for electroscopes, as those contrivances are called, which , 
are used for detecting the presence and examining the 
properties of electricity. A downy feather, a lock of 
cotton, or pith-balls,* are severally convenient substan- 
ces for electroscopes. To one of these, say a pith- 
ball, we will tie a fine linen thread, about nine inches 
long, and suspend it from the wire, as at a. By 
slightly wetting the thumb and finger and drawing the 

* The pith of elder, of corn stalk, or of dry stalks of the artichoke, is suit- 
able for this purpose. 

131. What different classes of phenomena does electricity exhib- 
it 1 Use of experiments. 

132. Describe the apparatus in Fig. 57. How is the tube excited 1 


thread through them, it becomes a good conductor, and 
the electroscope is therefore uninsulated. We will 
now take a thick glass tube and rub it with a piece of 
silk, (or a dry silk handkerchief,) by which means the 
! tube will be excited, and on approaching it towards 
! the electroscope, the pith-ball will be attracted towards 
! it, as at b, and may be led in any direction by shifting 
the position of the tube ; or if the tube be brought 
nearer, the ball will stick fast to it. We will next 
suspend two other balls, c and d, by silk threads, in 
! which case they will be insulated. If we now ap- 
proach the excited tube, the balls will first be attracted 
to it, but as soon as they touch it, they will fly off, and 
the tube when again brought towards them will no 
longer attract but will repel them, and they will mu- 
tually repel each other as in the figure ; and if the 
lock of threads, e, be electrified, they will also repel 
each other. A stick of sealing-wax excited and ap- 
plied to the electroscopes will produce similar effects, 
j But if we first electrify the ball with glass, and then 
| bring near it the sealing-wax, previously excited, it will 
j not repel the ball, as the excited tube does, but will first 
j attract it as though it were unelectrified, and then re- 
j pel it ; and now the excited glass tube will attract it. 
j Hence it appears that the glass and the sealing-wax, 
| when excited, produce opposite effects : what one at- 
! tracts the other repels. Each repels its own, but 
I attracts the opposite. Glass repels a body electrified 
I by itself, but attracts a body electrified by sealing-wax ; 
i and sealing-wax repels a body electrified by itself, but 
L attracts a body electrified by glass. In the figure, h 
represents two balls differently electrified, one by glass 
and the other by sealing-wax, and therefore attracting 
\ each other. This fact has led to the conclusion, that 

i. Effect when applied to the uninsulated ball to the insulated balls to 
i the threads. Describe the effects when sealing-wax is used when the 
, balls are differently electrified. What are the two kinds of electricity 1 


there are two kinds of electricity ; one excited by glass 
and a number of bodies of the same class, called the 
vitreous electricity, and the other excited by sealing. 
wax and other bodies equally numerous, of the same 
class with it, called the resinous electricity. Vitreous 
electricity, is sometimes called positive) and resinous 
electricity negative. 

133. The foregoing cases of electrical attractions 
and repulsions constitute important laws of electrical 
action, and are to be treasured up in the memory in the 
following propositions : 

First. An electrified body attracts all unelectrified 

Secondly. Bodies electrified similarly, that is, both 
positively or both negatively, repel each other. 

Thirdly. Bodies electrified differently, that is, one 
positively and the other negatively, attract each other. 

Fourthly. The force of attraction or repulsion is in- 
versely as the square of the distance ; that is, when two 
balls are electrified, the one positively and the other 
negatively, the force of attraction increases rapidly as 
they draw near to each other, being four times as great 
when twice as near, and a hundred times as great when 
ten times as near. Repulsion follows the same law; 
that is, when two balls are similarly electrified, it re- 
quires four times the force to bring them twice as near 
to each other, and a hundred times the force to bring 
them ten times as near as before. 

SEC. 2. Of Electrical Apparatus. 

134. Electrical machines afford the means of accu- 
mulating the electric fluid, so as to render its effects 
far more striking and powerful than they appear in the 
simple experiments already recited. The cylinder 

133. State the four laws of electrical attraction and repulsion 



.machine is represented in Fig. 58. Its principal 
i parts are the cylinder, the frame, the rubber, and the 

Fig. 58. 

prime conductor. The cylinder (A) is of glass, from 
eight to twelve inches in diameter, and from twelve 
to eighteen inches long. The frame (B B) is made 
of hard wood, dried and varnished. The rubber (C) 
consists of a leathern cushion, stuffed with hair like 
the pad of a saddle. This is covered with a black silk 
cloth, having a flap, which extends from the cushion 
over the top of the cylinder to the distance of an 
inch from the points of the prime conductor, to be 
mentioned presently. The rubber is coated with an 
amalgam, composed of quicksilver, zinc, and tin, which 
preparation has been found by experience to produce 

134. Describe the electrical machine the cylinder the frame 
the rubber the amalgam the prime conductor. 


a high degree of electrical excitement, when subjected 
to the friction of glass. The prime conductor (D) is 
usually a hollow cylinder of brass or tin, with rounded 
ends. It is mounted on a solid glass pillar, (a junk- 
bottle with a long neck will answer,) with a broad and 
heavy foot made of wood to keep it steady. The cyl- ; 
inder is perforated with small holes, for the reception 
of wires (c) with brass knobs. It is important in an- 
electrical machine, that the work should be smooth 
and free from points and sharp edges, since these have- 
a tendency to dissipate the fluid, as will be more fully 
understood hereafter. For a similar reason, the ma- 
chine should be kept free from dust, the particles of 
which act as points, and dissipate the electricity. 

135. By the friction of the glass cylinder against 
the rubber, electricity is produced, which is received* 
by the points, and thus diffused over the surface of the> 
prime conductor, and may be drawn from it by the> 
knuckle, or any conducting substance. In order to 
indicate the degree of excitement in the prime con- 
ductor, the Quadrant Electrometer is attached to it, as* 
is represented at E, Fig. 58. This electrometer iss 
formed of a semicircle, usually of ivory, divided into- 
degrees and minutes, from to 180. The index con- 
sists of a straw, moving on the center of the disk, and! 
carrying at the other extremity a small pith-ball. The- 
perpendicular support is a pillar of brass, or some con-j 
ducting substance. When this instrument is in a per- 
pendicular position, and not electrified, the index hangs 
by the side of the pillar, perpendicularly to the hori- 
zon; but when the prime conductor is electrified, it 
imparts the same kind of electricity to the index, re- 
pels it, and causes it to rise on the scale towards ai> 
angle of 90 degrees, which point indicates a full charge- 

135. How is the electricity produced 1 Describe the quadrant elec- 
trometer, and show how it indicates the degree of the charge. 


136. Let us now try a few experiments. If we 
(turn the machine one or two rounds, the prime con- 
Muctor will be charged, and the quadrant electrometer 
[will remain fixed at 90 degrees. We will first exam- 
Sine the conducting powers of different bodies. A glass 
'tube held in the hand and applied to the prime con- 
ductor will not cause the index of the electrometer to 
fall, because glass is a non-conductor of electricity ; 
but an iron rod thus applied, will cause the index to 
fall instantly, iron being a good conductor, and permit- 
ting the fluid readily to escape first to my hand, and 
through my person to the floor, and finally to the earth. 
On applying a knuckle to the prime conductor, we 
find, in the same manner, that the animal system is a 
!good conductor, as the fluid is instantly discharged and 
ithe index falls. On the other hand, a piece of sealing- 
jwax will not affect the index, and is therefore a non- 
iconductor. So, if we hold a lock of cotton by a silk 
jthread it will scarcely affect the electrometer, while if 
held by a linen thread, the fluid will be drawn off and 
the index will fall. It is very useful for the learner 
to try in this way the conducting powers of a great 
variety of bodies. Some he will find to affect the 
j electrometer very little, and he will thus know them 
!to be non-conductors ; others will instantly cause it 
!to fall, and are known as good conductors. Others 
'will cause the index to descend gradually, and are of 
; course imperfect conductors. These last, on being 
I moistened with the breath or wet with water, will in- 
dicate an increase of conducting power. A long stick 
* of wood, as a broom-handle, will be found to conduct 
1 with less power than a short stick of the same, and a 
large thread will conduct better than a small one. 

136. Experiments on the conducting powers of bodies glass iron 
the knuckle sealing-wax a silk thread. State the efl'ect of each 
, of these. What is the effect on conducting power produced by 
moisture by increasing the length or size of a bad conductor 1 



Fig. 59. 

Thus all the different circumstances affecting the con- 
ducting power, may be ascertained ; and upon the 
knowledge of these relative powers, depends the art 
of managing the electric fluid, whether in the form of 
common electricity or in that of lightning. 

137. The laws of attraction and repulsion may be 
verified by the aid of an electrical machine, much 
more strikingly than by the simple apparatus men- 
domed in Articles 132 and 133. If we hang a lock of 
hair to the prime conductor, on turning the machine 
the hairs will recede violently from each other, be- 
cause bodies similarly electrified repel each other. 
By placing light bodies, as paper images, locks of cot- 
ton, or light feathers, between one plate connected with 
the prime conductor and another which is uninsulated, 
as is represented in figure 59, (the upper plate being- 
hung to the prime conductor y ) 
the electrical dance may be per- 
formed. The images will first 
be attracted to the upper plate, 
but instantly imbibing the same 
electricity, they will be repelled 
by the upper and attracted by 
the lower plate on descending 
to the latter, they will give up 
their charge and return again 
to the upper plate to repeat the . 
process, thus performing a kind 
of dance, which when performed 
by little images of men and 
women, is often very amusing. 
Most electrical machines are furnished with a variety 
of apparatus for illustrating the principles of electrical 
attractions and repulsions, such as a chime of bells, 
the electrical horse-race, the electrical wind-mill, and 

137. Effect when a lock of hair is hung to the prime conductor ^ 
How is the electrical dance performed 1 



the like ; but these must be seen in order to be fully 
understood, and therefore their exhibition is left to the 

138. The Leyden Jar is a piece of ap- 
paratus used for accumulating a large Fig. 60. 
quantity of electricity. It consists of a 
glass jar coated on both sides with tin foil, 
except a space .on the upper end, within 
two or three inches of the top, which is either 
left bare, or is covered with a coating of var- 
: nish, or a thin layer of sealing wax. To 
the mouth of the jar is fitted a cover of hard 
j baked wood, through the center of which 
! passes a perpendicular wire, terminating 
I above in a knob, and below in a fine chain 
j that rests on the bottom of the jar. On presenting the 
knob of the jar near the prime conductor of an elec- 
trical machine, while the latter is in operation, a series 
of sparks pass between the conductor and the jar, 
which will gradually become more and more feeble, 
until they cease altogether. The jar is then said to 
be charged. If we now take the dis- 
charging rod, (which is a bent wire, 
armed at both ends with knobs, and in- , 
sulated by a glass handle, as in figure 
61,) and apply one of the knobs to the 
outer coating and bring the other to 
the knob of the jar, a flash of intense 
brightness, accompanied by a loud re- 
port, immediately ensues. If, instead 
of the discharging rod, we apply one 
hand to the outside of the charged jar, 
and bring a knuckle of the other hand to the knob of 
the jar, a sudden and surprising shock is felt, convul- 

Fig. 61. 

138. Define the Leyden Jar describe it how is it charged 1 
[ow discharged 1 How is the shock taken 1 


sing the arms, and when sufficiently powerful, passing 
through the breast. 

139. The outside and the inside of a Leyden Jar are 
always found in opposite states ; that is, if to the knob I 
connected with the inside we have imparted positive'; 
electricity, (as in the mode of charging already de-| 
scribed,) then the outside will be electrified in the same 
degree with negative or resinous electricity. Every 
spark of one sort of fluid that enters into the jar, drives 
off a spark of the same kind from the outside, and 
leaves that in the opposite state. And if the jar is in- 
sulated, (as when it stands on a glass support,) so that 
the electricity cannot pass from the outer coating, then 
it will take no charge. We may charge a jar nega- 
tively instead of positively, 'by grasping hold of the 
knob and presenting the outside to the prime conductor. 
The positive electricity that enters the outer coating, 
drives off an equal quantity of the same kind from the 
inside, which escapes through the body of the operator 
and leaves the inner coating negative. When the jar 
is thus charged, we must be careful to set it down on 
a.- glass support before withdrawing the hand; for if 
we place it on the table, which is a conductor, the elec- 
tricity will immediately rush from the outside to the 
inside, through the table, floor, and body of the opera- 
tor, and he will receive a shock. But if he sets the 
jar on a non-conducting support, no such communica- 
tion will be formed between the two sides of the jar, 
and consequently it will not discharge itself. 

The Electrical Spider forms a pleasing illustration 
of the different states of two jars, one charged posi- 
tively and the other negatively. It is contrived as fol- 
lows : Take a bit of cork and form a small ball of the 
size of a pea, for the body of the spider. With a 
needle, pass a fine black thread backward and for- 

139. In what state are the two sides of a charged jar 1 How may 
we charge a jar negatively 1 Why is it necessary to set it down on an 



Fig. 62. 

ward through the sides of the cork, letting the threads 
project from it half or three fourths of an inch on the 
opposite sides, to form the 
legs. Now suspend it from 
the center of the body by 
a fine silk thread, between 
two jars, one charged posi- 
tively and the other nega- 
tively, and placed on a table, 
as is represented in figure 
62. The spider will first 
be attracted to the knob of 
the nearest jar, will imbibe 
the same electricity, be re- 
pelled, and attracted to the 
knob of the other jar, from 
which again it will be repelled, and so will continue 
to vibrate back and forth between the two jars, until it 
has restored the equilibrium between them by slowly 
conveying to the inside of each jar the electricity of 
the inner coating of the other. 

Pointed conductors have a remark- 
able pow r er of drawing off and dis- 
sipating the electric fluid when it 
has accumulated. If we apply 
one hand to the outer coating of a 
charged jar, and with the other bring 
a needle towards the knob, it will 
silently draw off all the charge, 
without giving any shock. And if," 

j while we are charging a jar with the 

1 machine, we direct a pointed wire 
or a needle towards the machine, even at a much greater 
distance from it than the knob of the jar, the fluid will 

insulated support 1 Describe the electrical spider. Why does it 
vibrate from one jar to the other 1 Effect of points. 

Fig. 63. 


pass into the needle in preference to the jar. All 
apparatus, therefore, for confining electricity, requires 
to be free from sharp lines and points, and to terminate 
in round smooth surfaces. 

SEC. 3. Of Electrical Light and Heat. 

140. Electrical Light appears whenever the fluid is 
discharged in considerable quantities through a resist- 
ing medium. When electricity flows freely through 
good conductors, it exhibits neither light nor heat ; but 
if such conductors suffer any interruption, as in pass- 
ing through a small space of air, or even through an 
imperfect conductor, then light becomes manifest. We 
will suppose the experiment to be performed in a dark 
room, or in the evening, in a room very feebly lighted. 
A glass tube, rubbed with black silk, coated with a 
little electrical amalgam, will afford numerous sparks, 
with a slight crackling noise. A chain, hung to the 
prime conductor of a machine, will show a bright 
spark at every link. If we attach one end of the 
chain to the prime conductor, and hold the other 
end suspended by a glass tube, brushes or pencils 
of light will issue from various points along the 
chain. The spark seen in discharging the Leyden 
Jar, as in Article 138, is very intense and dazzling. 
Fig. 64. 

Figure 64 represents a glass cylinder, armed at each 
end with brass balls, and wound round, spirally, with 
a narrow strip of tin foil. At short intervals, small 
portions of the tin foil are cut out, so as to interrupt 

140. When does electrical light appear 1 When does electricity 
exhibit neither light nor heat 1 Experiment with a glass tube a 
chain a spiral tube 1 How may illuminated words be made to 
appear 1 


the circuit. Whenever a spark is passed through this 
apparatus, it appears beautifully luminous at every 
interruption in the tin foil. Words or figures of any 
kind may be very finely exhibited by coating a plate 
of glass with a strip of tin foil in a zigzag line, from 
one corner to the opposite corner, diagonally. Then 
with the point of a knife, small portions of the tin foil 
are nicked out in such a manner that the spaces 
thus left bare shall together constitute some word, as 
WASHINGTON. The spark, in passing through the tin 
foil, will meet with resistance at all the places where 
the metal has been removed, and will there exhibit a 
bright light. Thus an illuminated word will appear 
at every spark received from the machine. If the 
machine is not sufficiently powerful to afford a spark 
strong enough to overcome the resistance occasioned 
by so many non-conducting spaces, then the illu- 
minated word may be made to appear with great 
splendor, by making the plate form a part of the cir- 
cuit between the inside and the outside of a charged 
Leyden Jar. 

141. By means of the Battery, far more brilliant 
experiments may be performed than with a single jar. 
The Battery consists of a number of jars, twelve, for 
instance, so combined that the whole may be either 
charged or discharged at once. Large Leyden Jars, 
placed side by side in a box, standing on tin foil, which 
forms a conducting communication between the outer 
coatings, while the inner coatings are also in commu- 
nication by a system of wires and knobs, answer the 
same purpose as a single jar of enormous size, and are 
far more convenient. When the battery is charged, 
and a chain is made to form a part of the circuit 
between the outside and inside, on discharging it, the 
whole chain is most brilliantly illuminated. Rough 

141. The Battery of what does it consist 1 Describe it. How is 
a chain illuminated by the battery 1 Great power of some batteries. 


lightning rods sometimes present a similar appearance 
when struck during a thunder storm. Batteries are 
sometimes made of sufficient power to kill small animals, 
and even men. 

142. Heat, as well as light, attends the electric spark, 
although, except when the discharge is very powerful, 
as in the case of the battery, or of lightning, it is but 
feeble, sufficient to set on fire only the most inflammable 
substances. Alcohol and ether, two very inflammable 
liquids, may be fired by the spark, a candle may be 
lighted, and gunpowder exploded. It is, however, 
difficult to set powder on fire by electricity, unless the 
spark is very strong. 

143. The electric spark passes much more easily 
through rarefied air, than through air in its ordinary 
state. Thus, a spark which would not strike through 
the air more than four or five inches, will pass through 
an exhausted glass tube, four feet or more in length, 
filling all the interior with a soft and flickering light, 
somewhat resembling the Aurora Borealis. Hence, that 
phenomenon has been ascribed by some to electricity, 
though this is probably not its true explanation. 

144. In Thunder Storms, we see electricity exhibited 
in a state of accumulation far beyond what we can 
create by our machines, and producing effects propor- 
tionally more energetic. A cloud presents a conductor 
insulated by the surrounding air, in which, in hot 
weather, electricity collects and accumulates as it 
would upon a prime conductor of immense size. By 
sending up a kite armed with points, electricity may 
be drawn from such clouds, and made to descend by 
a wire wound round the string of the kite. We 

142. Does heat attend electricity 1 Give examples of bodies fired 
by it. 

143. How does the spark pass through rarefied air 1 Explain the 
appearance of the Auroral tube. 

144. How is electricity exhibited in thunder storms 1 Analogy be- 
tween a cloud and a prime conductor. How may lightning be drawn 


may easily direct it upon a prime conductor, or charge 
a Leyden Jar with it, and examine its properties as we 
should do in the case of ordinary electricity. By such 
experiments, it is found that the clouds are sometimes 
positively and sometimes negatively electrified. In 
thunder storms, the lightning is usually nothing more 
than the electric spark passing from one cloud to an- 
other differently electrified, as it passes between the 
outer and inner coating of the Leyden Jar. The flash 
appears in the form of a line, because it passes so 
swiftly, just as a stick, lighted at the end and whirled 
in the air, forms a circle of light. The motion of the 
electric fluid is, to all appearance, instantaneous. 
Thunder is the report occasioned by the rushing to- 
gether of the air, after it has been divided by the pas- 
sage of the lightning. The cracking* of a whip, as al- 
ready mentioned, is ascribed to the same cause. The 
lash divides the air into two parts, which forcibly rush 
together and occasion the sound. When a thunder- 
clap is very near us, the report follows the flash almost 
instantly, and such claps are dangerous. In all cases, 
the lightning and the thunder actually occur at the 
same moment, but when the discharge is at some dis- 
tance from us, the report is not heard till some time 
after the flash ; for the light reaches the eye instanta- 
neously, but the sound travels with comparative slow- 
ness, moving only about a mile in five seconds. We 
may, therefore, always know nearly how distant a 
thunder cloud is, by counting the number of seconds 
between the flash and the report, and allowing the fifth 
of a mile (or, more accurately, 1,130 feet) to a second. 
(See Art. 124.) 

145. Sometimes lightning, instead of passing from 

from the clouds *? How is the flash produced in thunder storms 1 
Why does it leave a bright line 1 What is thunder 1 How produced 1 
Why are the flash and the report sometimes together and sometimes 
separate 1 


cloud to cloud, discharges itself into the earth, and 
then strikes objects that come in its route, as houses, 
trees, animals, and sometimes man. As electricity 
always selects, in its passage, the best conductors, Dri 
Franklin first suggested the idea of protecting our 
dwellings by means of Lightning Rods. If these are: 
properly constructed, the lightning will always take its < 
passage through them in preference to any part of the 
house, and thus they will afford complete protection to 
the family. Sharp metallic points were observed by 
Dr. Franklin to have great power to discharge elee-1 
tricity from either a prime conductor or a Leyden Jar, 
and this suggested their use in lightning-rods. Metals, s 
also, being the best conductors of electricity, would 
obviously afford the most proper material for the bodyj 
of the rod. 

There are three or four conditions in the construc- 
tion and application of a lightning-rod, which are es- ' 
sential to insure complete protection. The rod must 1 
not be less than three-fourths of an inch in diameter 
it must be continuous throughout, and not interrupted 
by loose joints it must terminate above in one or more 
sharp points of some metal, as silver, gold, or platina, ] 
not liable to rust it must enter the ground to the depth 
of permanent moisture, which will be different in dif-J 
ferent soils, but usually not less than six feet. A rod 
thus constructed will generally protect a space every " 
Way equal to twice its height above the ridge of the 
house. Thus, if it rises fifteen feet above the ridge, it 
will protect a space every way from it of thirty feet. 
It is usually best to apply the rod to the chimney of 
the house or, if there are several chimneys, it is best 
to select one as central as possible. The kitchen 

145. What happens when lightning strikes to the earth 1 Lightning- 
rods influence of points and conductors power of metals size of 
the rod to be continuous how terminated above and below 1 How 
much space will a rod protect 1 How applied to a house 1 What is 


chimney, being usually the only one in which fires are 
maintained during the season of thunder storms, re- 
quires to be specially protected, since a column of 
smoke rising from a chimney is apt to determine the 
course of the lightning in that direction. If, therefore, 
the lightning-rod is attached to some other chimney of 
the house, either a branch should proceed from it up 
the kitchen chimney, or this should have a separate 
rod. As lightning, in its passage from a cloud to the 
-earth, selects tall pointed objects, it often strikes trees, 
and it is, therefore, never safe to take shelter under trees 
during a thunder storm. Persons struck down by light- 
ning are sometimes recovered by dashing on repeated 
buckets of water. 

SEC. 4. Of the Effects of Electricity on Animals. 

146. When we apply a Fig. 65. 

knuckle to the prime conduct- 
or of an electrical machine, 
and receive the spark, a sharp 
and somewhat painful sensa- 
tion is felt. If we receive the- 
charge of a Leyden Jar, a 
shock is experienced which is 
more or less severe, accord-, 
ing to the size and power of J 
the jar. A battery gives a 
shock still more severe, and 
it may be even dangerous. 
Lightning, it is well known, 
sometimes prostrates and kills men and animals. A 
convenient method of taking the shock, is to charge a 

said of the kitchen chimney *? May we take shelter under trees 1 
How to^ restore people struck by lightning 1 

146. Sensation to the knuckle effects of a jar of a battery. What 
is a convenient mode of .taking the shock 1 Sensations produced by 



quart jar, place it on a table, and grasping in each hand 
a metallic rod, apply one rod to the outside of the jar, 
and touch the other to the knob connected with the in- 
side. If the charge is feeble, it will be felt only in the 
arms ; if it is stronger, it will be felt in the breast ; 
and it may be sufficiently powerful to convulse the 
whole frame. Any number of persons may, by taking 
hold of hands, all receive the shock at the same instant. 
The first must touch the outside, and the last the knob 
of the jar. Whole regiments have been electrified at 
once in this way. 

147. Electricity is sometimes employed medicinally, 
and is thought to afford relief in various diseases. It 
may be applied either to the whole system at once, or 
to any individual part, by making that part form a por- 
tion of the communication between the inside and the 

outside of a jar. Or the 
Fig. 66. fluid may be taken in a 

milder form by means 
of the Electrical Stool. 
This is a small stool, 
resting on glass feet. 
The patient stands or 
sits on the stool, and 
holds a chain connect- 
ed with the prime con- 
ductor, while the ma- 
chine is turned. This 
produces an agreeable 
excitement over the 
whole system : the hair stands on end ; sparks may 
be taken from all parts of the person, as from a prime 
conductor ; and the patient may communicate a slight 

a feeble charge by a strong by a powerful charge 1 How may 
any number of persons be electrified at once 1 

147. How is electricity employed medicinally! How by means 
of the electrical stool 7 


I shock to any one that comes near him, or may set on 
fire ether and other inflammable substances, by merely 
touching them with a rod, or pointing toward them. 

148. Several fishes have remarkable electrical pow- 
ers. Such are the Torpedo, the Gymnotus, and the 
Silurus. The Gymnotus, or Surinam eel, is found in 
the rivers of South America. Its ordinary length is 
from three to four feet ; but it is said to be sometimes 
twenty feet long, and to give a shock that is instantly 
fatal. Thus, it paralyzes fishes, which serve as its 
food, and in the same manner it disables its enemies 
and escapes from them. By successive efforts, elec- 
trical fishes exhaust themselves. In South America, the 
natives have a method of taking them, by driving wild 
horses into a lake where they abound. Some of the 
eels are very large, and capable of giving shocks so 
powerful as to disable the horses ; but the eels them- 
selves are so much exhausted by the process, as to be 
easily taken. 




149. AMONG the ores of iron, there is found an ore 
of a peculiar kind, which has the power of attracting 
iron filings, and other forms of metallic iron, and is 
called the loadstone. This power can be imparted to 
bars of steel, which are denominated magnets. The 
unknown power which produces the peculiar effects 
of the magnet, is called magnetism. This name is 

148. What of electrical rishes 1 Give an account of the Gymno- 
tus. How do^the natives take electrical fishes in South America'? 

149, What is the loadstone, and magnets 1 Define Magnetism 




also applied, as at the head of this chapter, to that 
branch of Natural Philosophy which treats of the 
magnet. Magnetic bars are thick plates of iron or 
steel, commonly about six inches long. If a magnetic 
bar be placed among iron filings, they will arrange 
themselves around a point at each end, forming tufts, 

Fig. 67. 

as is shown in figure 67. These two points are called 
the poles, and the straight line that joins them, the 
axis of the magnet. If we suspend, by a fine thread, 
a small needle, and approach toward it either poles 
of a metallic^ bar, the needle will rush toward itj 
and attach itself strongly to the pole. By rubbing 
the needle on one of the poles of the magnet itl 
will itself imbibe the same power of attracting iron, 
and become a magnet, having its poles. If we now 
bring first one pole of the mag- 
Fig. 68. Fig. 69. netic bar toward the needle, andv 
1 then the other pole, we shall find 
that one attracts, and the other re-^ 
pels the needle. Figure 68 repre- 
sents two large sewing needles, 
magnetized, and suspended by fine 
threads. On approaching the north 
pole of a 'magnetic bar to the north 
poles of the needles, they are 
forcibly repelled ; but on apply- 
ing the south pole of a bar, as in 
figure 69, the north poles of the 
edles are attracted toward it. 

two senses in which the word is used. What are magnetic bars 1 
What are the poles the axis 1 How may a needle be magnetized 1 
How are its properties changed by this process 1 


150. Let us suppose that the long needle represent- 
ed in figure 70, has been rubbed on a magnet, so as to 
imbibe its properties, or to 
become magnetized ; then, on 
balancing it on a pivot, it will s 
of its own accord place itself 
in nearly a north and south 
line, and return forcibly to 
this position when drawn 
aside from it. This property 
is called the directive, while the other is called the 
attractive, property of the magnet. That end which 
points northward, is called the North Pole of the 
magnet, and that end which points southward, is 
called the South Pole. Every magnet has these two 
poles, whatever may be its size or shape. A mag- 
netic bar has usually a mark across one end, to de- 
note that it is the north pole, the other, of course, 
being the south pole. If the north pole of a bar be 
brought toward the north pole, N, (Fig. 70,) of the 
needle, it will repel it, and the more forcibly in pro- 
portion as we bring it nearer to N. On the contrary, 
if the north pole of the bar be brought toward the 
south pole S of the needle, it will attract it. Also, 
if we present the south pole of the bar first to one pole 
of the needle, and then to the other, we shall find that 
the bar will repel the pole of the same name with its 
own, and attract its opposite. These facts are ex- 
pressed by the proposition that similar poles repel, and 
opposite poles attract each other. When a magnetic 
bar is laid on a sheet of paper, and iron filings are 

150. Explain the directive property. "Which is the north and 
which the south pole 1 How is the north pole distinguished 1 Ef- 
fect when the north pole of the bar is brought near the north pole 
of the needle when the north pole is brought toward the south 

Eole 1 State the general fact, w hat takes place when a magnetic 
ar is placed among iron filings 1 


sprinkled on it, they will arrange themselves in curves 
around it, as in figure 71. 

Fig. 71. 

151. The magnetic needle, when freely suspended, 
seldom points directly to the pole of the earth, but its 
deviation from that pole, either east or west, is called 
the variation of the needle. A line drawn on the sur- 
face of the earth, due north and south, is called a me- 
ndian line. The needle usually makes a greater or| 
less angle with this line. Its direction is called the 
magnetic meridian, and the place on the earth to which 
it points, is called the magnetic pole. The earth has 
two magnetic poles, one in the northern, the other in 
the southern hemisphere. The north magnetic pole 
is in the part of North America lying north of Hud- 
son's and west of Baffin's Bay, in latitude 70. The 
variation of the needle is different in different coun- 
tries. In Europe, the needle points nearly N. W. 
and S. E. ; while in the United States it deviates no- ' 
where but a few degrees from north and south ; and 
along a certain series of places, passing through West- 
ern New York and Pennsylvania, the variation is noth- 
ing ; that is, the needle points directly north and south. 
At the same place, moreover, the variation of the 
needle is different at different periods. For a long 
series of years, the needle will slowly approach the 

151. "What is meant by the variation of the needle 1 What is a 
meridian line 1 the magnetic meridian *? Situation of the north mag- 
netic pole ] How is the variation of the needle in Europe 1 How in 
the United States 1 Where does the line of no variation run 1 How 



! North pole, come within a certain distance of it, and 
then turn about and again slowly recede from it. At 
Yale College, the variation in 1843, was 6i degrees 
West, and is increasing at the rate of 4J minutes a year. 

152. A needle first balanced on its center of gravity, 
and then magnetized, no 

longer retains its level, but Fig. 72. 

it points below the horizon, 
making an angle with it, 
called the Dip of the needle. 
The dipping needle is 
shown in figure 72, adapted . 
to a graduated circle in or- r 
der to indicate the amount of \ 
the depression, and is some- 
times fitted with screws and 
a level to adjust it for obser- 
vation . The dip of the nee- 
dle varies very much in dif- 
ferent parts of the earth, being in general least in the 
equatorial, and greatest in the polar regions. At Yale 
College, it is about 73 degrees, being greater than is 
exhibited in the figure. 

153. The directive property of the needle has two 
most interesting and important practical applications, 
in surveying and navigation. The compass needle, 
in order to keep it at a horizontal level, and prevent its 
dipping, has a counterpoise on one side, which exactly 
balances the tendency to point downward. By the 
aid of this little instrument, lands are measured, and 
boundaries determined ; the traveller finds his way 

does the variation change at any given place 1 How is it at New 
Haven 1 

152. What is the Dip of the needle 1 Describe figure 72. "Where 
is the dip of the needle greatest 1 Where least 1 Its amount at 
Yale College 1 

153. What are the two leading applications of the needle 1 How is 
the compass needle kept from dipping ! To what uses is it applied 1 



through unexplored forests and deserts ; and mariners 
guide their ships through darkness and tempests, and 
across pathless oceans. 

154. There are various methods of making compass 
needles, or artificial magnets. Soft iron readily receives 
magnetism, but as readily loses it ; hard steel receives 
it more slowly, but retains it permanently. It is a 
singular property of a magnet, whether natural or 
artificial, that, like virtue, it loses nothing by what it 
imparts to another. In fact, such an exercise of its 
powers is essential to their preservation. The strongest 
magnet, if suffered to remain unemployed, gradually 
loses power. Magnets, therefore, and, loadstones, are 
kept loaded with as much iron as they are capable of 
holding, called their armature. If we simply rub a 
penknife on one pole of a magnet, we render it magnetic, 
as will be indicated by its taking up iron filings or 
sewing needles. Magnetism is most readily imparted 
by a bar, when both its poles are made to act together. 
This is done by giving the bar the form of a horse-shoe, 
as in figure 73. To magnetize a needle, we lay it flat 

on a table, and place the 
Fig. 73. two poles of the horse- 

shoe magnet near the 
middle, and rub it on the 
needle, backward and 
forward, first toward one 
end and then toward the 

other, taking care to pass over each half of it an equal 
number of times. The needle may then be turned over, 
and the same process performed on the other side, when 
it will be found strongly and permanently magnetized. 

154. How is the compass needle made 1 "What is said of soft iron 
and hard steel 1 How is the strength of the magnet affected by 
action or inaction 1 What is the armature 1 How to magnetize a 
penknife. Why is a bar bent into the horse-shoe form 1 How to 
magnetize a needle with it. 



155. OPTICS is that branch of Natural Philosophy 
which treats of Light. Light proceeds from the sun, a 
lamp, and all other luminous bodies, in every direction, 
in straight lines, called rays. If it consists of matter, 
its particles are so small as to be incapable of being 
weighed or measured, many millions being required 
to make a single grain. Some bodies, as air and glass, 
readily permit light to pass through them, and are 
called transparent ; others, as plates of metal, do not 
permit us to see through them, and are called opake. 
Any substance through which light passes, is called 
a Medium. Light moves with the astonishing velocity 
of 192,500 miles in a second. It woulcj cross the 
Atlantic Ocean in the sixty-fourth part of a second, 
and in the eighth part of a second, would go round the 
earth. When light strikes upon bodies, some portion 
of it enters the body, or is absorbed, and more or less 
of it is thrown back, and is said to be reflected ; when 
it passes through transparent bodies, it is turned out 
of its direct course, and is said to be refracted. The 
light of the sun consists of seven different colored 
rays, which, being variously absorbed and reflected 
by different bodies, constitute all the varieties of colors. 
Light enters the eye, and forming within it pictures 
of external objects, thus gives the sensation of vision. 
The knowledge of the properties of light, and the 
nature of vision, has given rise to the invention of 
many noble and excellent instruments, which afford 

155. Define Optics terms rays, transparent, opake, and medium. 
When is light said to be reflected 1 When refracted 1 Of what do the 


wonderful aid to the eye, such as the microscope and 
the telescope. Let us examine more particularly these 
interesting and important subjects, under separate 

SEC. 1. Of the Reflexion and Refraction of Light. 

156. When rays of light, on striking upon some body, 
are turned back into the same medium, they are said 
to be reflected. Smooth polished surfaces, like mirrors 
and wares of metal, reflect light most freely of any, and 
hence their brightness. Most objects, however, are 
seen by reflected light ; few shine by their own light. 
Thus, the whole face of nature owes its brightness and 
its various colors to the light of the sun by day, and \ 
to the light of the moon and stars by night. The rays 
that come from these distant luminaries, fall first upon 
the atmosphere, and are so reflected and refracted from 
that as to light up the whole sky, which, were it not for 
such a power of scattering the rays of light that fall 
upon it, would be perfectly black. On account of the^ 
transparency of the atmosphere, the greater part of 
the sun's rays pass through it, and fall upon the surface 
of the earth, and upon all objects near it. These reflect 
the light in various directions, and are thus rendered 
visible by that portion of the light which proceeds from 
them to the eye. 

157. When a ray of light strikes upon a plane 
surface, the angle which it makes with a perpendicular 
to that surface, is called the angle of incidence, and the 
angle which it makes with the same perpendicular, 
when reflected, is called the angle of reflexion. The 

sun's rays consist 1 To what inventions has the study of Optics 
given rise *? 

156. When are rays of light said to be reflected *? By what light 
are most objects seen 1 Show how the atmosphere and most things 
on the earth are illuminated. 

157. Define the angle of incidence and of reflexion. Equality be- 



angle of reflexion is equal to the angle of incidence. 
Thus, a ray of light, A C, striking upon a plane mir- 
ror, M N, at C, will be 
reflected off into the line 
C B, making the angle of A 
incidence, MCA, equal N 
to the angle of reflexion, 
N C D. It is not neces- 
sary that the surface on 
which the light strikes/ 
should be a continuedl 
plane ; the small part of M 
a curved surface, on which 
a ray of light falls, may be considered as a plane, 
touching the curve at that point, so that the same law 
of reflexion holds in curved as in plane surfaces. 
Now the grains of sand on a sandy plain, present sur- 
faces variously inclined to each other, which scatter 
the rays of the sun in different directions, many of 
which enter the eye, and make such a region appear 
very bright ; while a smooth surface, like a mirror, or 
a calm sheet of water, reflects the light that falls on it 
chiefly in one direction, and hence appears bright only 
when the eye is so situated as to receive the reflected 
beam. Thus, the ocean appears much darker than 
the land, except when the sun shines upon it at such 
an angle as to throw the reflected beam directly to- 
ward the eye, as at a certain hour of the morning or 
evening, and then the brightness is excessive. 

158. An object always appears in the direction in 
which the last ray of light from it comes to the eye. 
Thus, we see the sun below the surface of a smooth 
lake or river, because every ray of light, being reflect- 
ed from the water as from a mirror, comes to the eye 

tween these two angles. Explain figure 74. Does the same law 
hold for curved surfaces 1 Which appears darkest, the ocean or 
land, in the light of the sun 1 



in the direction in which the image appears ; and if 
the light of a star were to change its direction a hun- 
dred times in coming through the atmosphere, we should? 
see the star in the direction of the last ray, in the same 
manner as if none of the other directions had existed. 
This principle explains various appearances presented 
by mirrors, of which there are three kinds plane, 
concave, and convex. 

159. A common looking-glass furnishes an example 
of a Plane Mirror. If we place a lamp before it, rays 
of light are thrown from the lamp upon every part of 
the mirror, but we see the lamp by means of those few 
of the rays only which are reflected to the eye ; all 
the rest are scattered in various quarters, and do not 
contribute at all to render the object visible to a spec- 
tator at any one point, although they would produce, 
in like manner, a separate image of the lamp wherever 
they entered an eye so situated as to receive them. 
Hence, were there a hundred people in the room, each 
would see a separate image, and each in the direction 
in which the rays came to his own eye. We will sup- 

Fig. 75. 

pose M N to be the looking-glass, having a harp placed 

158. In what direction does an object always appear 1 Example 
in the sun in a star. What are the three kinds of mirrors 1 

159. Explain how the image is formed in a plane mirror. What 
rays only enable us to see the image 1 Explain Fig. 75. How far 

OPTICS. 155 

before it, and the eye of thev spectator at D. Of all 
the rays that strike on the glass, the spectator will see 
the image by those only which strike the mirror in 
such a direction, A B, that when reflected from the 
mirror at the same angle on the other side, they shall 
enter the eye in the direction B D. The image will 
appear at C, and will be just as far behind the mirror as 
the harp is before it. This last principle is an import- 
ant one, and it must always be remembered, that every 
point in an object placed before a plane mirror, will 
appear in the image just as far behind the mirror as 
that point of the object is before it ; so that the image 
will be an exact copy of the object, and just as much 
inclined to the mirror. We learn, also, the reason 
why objects appear inverted when we see them re- 
flected from water, as the surface of a river or lake, 
since the parts of the object most distant from the wa- 
ter, that is, the top of the object, will form the lowest 
part of the image. 

160. If we take a looking-glass and throw an image 
of the sun on a wall, on turning the mirror round we 
shall find that the image moves over twice as many 
degrees as the mirror does. If the image is at first 
thrown against the wall of a room, horizontally, (in 
which case the mirror itself would be perpendicular to 
the horizon,) by turning the mirror through half a 
right angle, the place of the image would be changed 
a whole right angle, so as to fall on the ceiling over 
head. A common table-glass, which turns on two 
pivots, being placed before a window when the sun is 
low, will furnish a convenient means of verifying this 

is the image behind the mirror 1 How far is each point in the im- 
age behind the mirror 1 Why do objects appear inverted when re- 
flected from water 1 

160. If a mirror be turned, how much faster does the image move 
than the mirror 1 State how the experiment is performed. 



161. A Concave Mirnor collects rays of light. If 
we hold a small concave shaving-glass, for instance, 
toward the sun, it will collect the whole beam of light 
that falls upon it into one point, called the focus. 
. Figure 76 will give some idea oft 
Fig. 76. the manner in which parallel rays 

strike a concave mirror, converge 
^ to a focus, and then diverge. The 
^ angle of reflexion is equal to the 
j angle of incidence here, as well 
^1 as in a plane mirror ; but the 
perpendicular to a curved surface 
is the radius of the circle of which the curve is a 
part. Thus, the line C B is the radius of the con- 

Fig. 77. 

cave mirror, M N, and, in a circle, every radius is 
perpendicular to the surface. The sun's rays are 
parallel to each other, or so nearly so, that they may 
be considered as parallel ; and when rays fall upon 
the mirror, in the lines A B and E G, they are re- 
flected on the other side of the perpendiculars, meet- 
ing in a common focus, F, which point is called the 
focus of parallel rays. Into this point, or a small 
space around it, a concave mirror will collect a beam 

161. What is the office of a concave mirror 1 Experiment with a 
shaving-glass. What forms the perpendicular to a concave surface T 
What is the point called where parallel rays are collected 1 What \a 

OPTICS. 157 

; of the sun, increasing in heat in the same proportion 
| as the illuminated space at F is less than the whole 
surface of the mirror. In large concave mirrors, 
the heat at the focus often becomes very powerful, 
iso as not only to set combustibles on fire, but even 
jto melt the most infusible substances. Hence the 
iname focus, which means a burning point. If a lamp 
iis placed at F, the rays of light proceeding from it in 
the lines F G and F B, will strike upon the mirror 
and be reflected back into the parallels, G E and B A. 
We shall see hereafter how useful this property of 
;concave mirrors, to collect parallel rays of light into 
a focus, is in the construction of that most noble of 
instruments, the telescope. 

A Convex Mirror, on the other ha^id, separates rays 
lof light from each other, still observing the same law, 
that of making the angle of incidence equal to the 

/ingle of reflexion. In figure 78, the parallel rays, 
jA B, D, E F, are represented as falling on a convex 
mirror, ^M N. A B and E F, being reflected to the 
other sides of the radii, C B and C F, are separated 

Ssaid of the heat at the focus 1 When a lamp is placed in the focus, 
now will its light be reflected? 




from each other, and form the image at I, which is 
called the imaginary focus of parallel rays, because, at 
this point, the parallel rays that fall upon the mirror 
seem to meet in a focus behind the mirror, and to 
diverge again into the lines B G and F H. 

162. Whenever the rays of light from the different 
parts of an object cross each other before forming the 
image, the image will be inverted. It is manifest from 
figure 79, that the light by which the' top of the object 

is represented forms the bottom of the image, and the 
light from the bottom of the object forms the top of the 
image, the two sets of rays crossing each other at the 
hole in the screen. It is always essential to the dis- 
tinctness of an image, that the rays which proceed 
from every point in the object, should be arranged in 
corresponding points in the image, and should be un- 
accompanied by light from any other source. Now a 
screen like that in the figure, when interposed, per- 
mits only those rays from any point in the object that 
are very near together and nearly parallel to each other, 
to pass through the opening, after which they continue 
straight forward and form the corresponding point of 
the image ; while rays coming from any other point in 
the object cannot fall upon the point occupied by the 

162. In what case will the image appear inverted 1 Explain from 
Fig. 79. What is essential to the distinctness of an image 1 What 
rays only does the screen permit to ] 



jformer pencil, but each finds an appropriate place of 
its own in the image, and all together make a faithful 
I representation of the object. 

163. Concave mirrors form images of objects, by 

collecting the rays from each point of the object into 

j corresponding points in the image, unaccompanied by 

, rays from any other quarter. If the object be nearer 

Fig. 80. 

Fig. 81. 


than the focus, as in figure 80, a magnified image ap- 
pears behind the mirror, and in its natural position ; 
but if the object be between the focus and the center, 
the image is before the mirror, on the other side of the 
center, larger than the object,' 
and inverted, as it is in fig- 
ure 81, where the small ar- 
row, A B, situated between 
the focus and the center of 
the mirror, is reflected into 
the image a b, inverted and 
larger than the object. These 
cases may be verified in a 
dark room, by placing a lamp 
at different distances from a . 
concave mirror. As such : 
mirrors form their images in 
the air without any visible 

163. How do mirrors form images 1 When the object is 
nearer the mirror than the focus, how does the image appear! 
How when it is farther than the focus *. How may these cases be 



support, they have sometimes been employed by jug. 
glers to produce apparitions of ghostly figures, drawn 
swords, and the like, which were made to appear in 
terrific forms, while the apparatus by which they were 
produced, was entirely concealed from the spectators. 

Fig. 82. 

A convex mirror gives a diminished image of any object 
placed before it, representing it in its natural position, 
and behind the mirror, as in figure 82. 

164. Refraction is the change of direction which light 
undergoes by passing out of one medium into another. 
When light passes out of a rare me- 
dium, like air, into a dense medium, 
like water, it is turned toward a per- 
pendicular ; when it passes out of a 
dense into a rare medium, it is turn- 
edfrom a perpendicular. When the 
ray of light, B C, passes out of air in- 
i to water, it will not proceed straight; 
in forward in the line C F, but will go> 
1 in the line C E, nearer to the per- 
pendicular, C H ; and light proceeding from an object 
under water at E would, on passing into the air at C, 
turn from the perpendicular into the line C B. Since 

verified 1 "What use is made of concave mirrors by jugglers'? How 
do convex mirrors represent objects 1 

164. Define refraction. How is light reflected by passing out of 
air into water out of water into air 1 Explain Fig. 83. Also, Fig.. 




Fig. 84. 

objects always appear 

| in the direction in which 

| the light finally comes 
to the eye, the place of 
an image is changed by 
its light passing through 
a refracting medium be- 
fore it reaches the eye. 
Fig. 84 represents a 
bowl with a small coin 
at the bottom. An eye 
situated as in the figure, would not see the coin ; but, 
on turning water into the bowl, the coin becomes vis- 
ible at B, because the light proceeding from the coin is 
bent toward the eye in passing out of the water. For 
a similar reason, an oar in water appears bent, the part 
immersed being elevated by refraction. The bottom 
of a shallow river appears higher than it really is, and 
people have been drowned by attempting to ford a river 
which, from the effect of refraction, appeared less deep 
than it was. 

165. The Multiplying Glass 
shows as many images of an 
object as there are surfaces, 
since each surface refracts the 
light that falls upon it, in a 
different angle from the others ; 
of course the rays meet the 
eye in the same number of 
different directions, and the 
object appears in the direction 
of each. The candle at A, Fig. 85, sends rays to 
each of the three surfaces of the glass. Those which 
fall on it perpendicularly, pass directly through the 

84. Why an oar in water appears bent 1 How does refraction affect 
the apparent depth of a river 1 

165. Describe the multiplying glass, and explain its effects. 

Fig. 85. 



glass to the eye, without change of direction, and form 
one image in its true place at A. But the rays which 
fall on the two oblique surfaces, have their directions 
changed both in entering and in leaving the glass, 
(as will be seen by following the rays in the figure,) 
so as to meet the eye in the directions of B and C. 
Consequently, images of the candle are formed, also, 
at both these points. A multiplying glass has> 
usually a great many surfaces inclined to one another, 
and the number of images it forms is proportionally 

166. This property of light the power of having 
its direction changed by refraction is converted to. 
very important and inter- 
esting uses by means of 
LENSES. A lens is exem- 
plified in a common sun- 
glass, (or even in a spec- 
tacle-glass,) and is either 
convex or concave. Con- 
vex lenses, like concave 
mirrors, collect rays of 
light. In Fig. 86, the parallel rays, A a and C c y 
are collected along with the central ray (which be- 
ing perpendicular to the surfaces of the lens, suffers, 
no refraction) into a common focus in F. If I hold a 
sun-glass, or a pair of convex spectacles toward the 
sun, the whole beam that falls upon the glass will be 
collected into a small space, forming a bright point, 
or focus, at a certain distance from the lens on the 
side opposite the sun, where it may be received on a 
screen or sheet of white paper. A concave lens, like 

166. To what important and interesting uses is the power of light 
to undergo refraction converted 1 What instruments are used Tor 
this purpose 1 What is the office of a convex lens 1 Describe Fig,. 
S6. Examples in a sun-glass and spectacles. 



i a convex mirror, separates rays of light. Thus, in 

Fig. 87, the solar beam 
Fig. 87. is spread over a greater 

space on the screen than 
the size of the lens, indi- 
eating that the rays are 
separated from each other 
by passing through the 
lens. Hence, concave 
lenses do not form images 
as convex lenses do, and 
are therefore but little employed in the construction of 
optical instruments. 

167. A convex lens, like a concave mirror, forms 
an image of an object without, by collecting all the 
pencils of rays that proceed from every point of the 
object and fall upon the lens, into corresponding points 
#t the place of the image. The image is inverted 

Fig. 88. 


because the pencils of rays cross each other, those 
from the top of the object going to the bottom of the 
image, and those from the bottom going to the top. 
In the figure, the central ray of each pencil (called the 
axis) and the extreme rays are represented. The ex- 

167. How does a convex lens form an image 1 Why is the image 
inverted 1 What is the axis of a pencil of rays 7 Where do the 
axes cross each other 7 Great number of rays that proceed from 
every point in the object. 


treme rays cross each other in the center of the lens,, 
and thus necessarily produce an inverted image ; but 
we must conceive of a great number of rays proceed-, 
ing from every point in the object, and each pencil- 
covering the whole lens, which collects them severally 
into distinct points, each occupying a separate place ' 
in the image. 

168. If we place a lamp in the focus of a lens, the 
rays that proceed from it and pass through the lens, 
go out parallel, and will never come to a focus on the 
other side, so as to form an image. But if we remove 
the lamp farther from the lens, so as to make the rays t 
fall upon the lens in a state less diverging, then it will ; 
collect them into a distinct image on the other side, J 
which image will be large in proportion as it is morel 
distant from the lens. As the object is withdrawn 
from the lens, the image approaches it ; when they are ) 
at equal distances from the lens, they are equal in size ; 
but when the object is farther from the lens than the \ 
image, the image is less than the object. These prin- 'I 
ciples lead to an understanding of those interesting f 
and wonderful instruments, the Microscope and the * 
Telescope, to which our attention will hereafter be di- ' 

SEC. 2. Of Colors. 

169. The philosophy of colors has been unfolded 
chiefly by means of the Prism. A Prism is a trian- 
gular piece of glass, usually four or five inches long, 
presenting three plane smooth surfaces. When we 
look through the prism, all external objects appear in 

168. How do the rays go out when a lamp is placed in the focus 1 
How when the lamp is farther from the lens than the focus 1 How 
is the size of the image affected by its distance from the lens 1 How 
is the image changed by withdrawing the lamp! 

169. By what instrument has the philosophy of colors been unfold- 
ed 1 Define a prism. What appearances does it present when we 



the most brilliant hues, diversified by the various colors 
of the rainbow. The reason of this is, that light consists 
of seven different colors, which, when in union with 
each other, compose white light ; but when separated, 
appear each in its own peculiar hue. The different 
colors are as follows violet, indigo, blue, green, yellow, 
orange, red. The prism separates the rays of solar 
light, in consequence of their having the property of 
I undergoing different degrees of refraction in passing 
i through it, the violet being turned most out of its course 
and the red least, and all the others differing among 
themselves in this respect, as is shown in the following 
diagram. E F represents the window shutter of a dark 

Fig. 89. 


room, through a small opening in which a beam of solar 
rays shines. They fall on the prism, ABC, and are 
refracted, by which they are turned upward, but in 
different degrees, the red least and the violet most. By 
this means they are separated from each other, and lie 
one above another on the opposite wall, constituting 

look through it 1 Why 1 Seven colors of the spectrum. "Why 
does the prism separate the different colors 1 Explain Fig. 89. How 
may \ve recompose the spectrum into white light 1 


the beautiful object called the solar spectrum. We 
may now introduce a double convex lens into thq: 
spectrum, just behind the prism, and collect all the 
rays which have been separated by the prism, and they 
will recompose white light. The elongated spectrum 
on the wall, presenting the seven primary colors, will 
vanish, and in the place of it will appear a round image 
of the sun as white as snow. 

170. We may now learn the reason why so many 
different colors appear when we look through the prism. 
The leaves of a tree, for example, seem to send forth 
streams of red light on one side and of violet on the 
other. The intermediate colors lap over, and partly 
neutralize each other, while on the margin each color 
exhibits its own proper hue. 

171. The rainbow owes its brilliant colors to the 
same cause, namely, the production of the individual 
colors that compose solar light, in consequence of the 
separation they undergo by refraction in passing through 
drops of water. Although drops of water are small 
objects, yet rays of light are still smaller, and have 
abundant room to enter a drop of water on one side, to 
be reflected from the opposite surface, and to pass out 
on the other side, as is represented in the following 
figure. The solar beam enters the drop of rain, and , 
some portion (a very small portion is sufficient) being 
refracted to B, then reflected and finally refracted 
again in leaving the drop, is conveyed to the eye of. 
the spectator. As in undergoing these two refractions, 
some rays are refracted more than others, consequently 
they are separated from each other, and coming to the 
eye of the spectator in this divided state, produce each 

170. Why do so many different colors appear when we look 
through the prism 1 Explain the appearance of the leaves of a tree. 

171. To what does the rainbow owe its colors 1 Explain how the 
separation of colors is produced. To what part of the bow does the 
line pass which joins the sun and the eye of the spectator 1 How high 

OPTICS. 167 

its own color. The spectator stands with his back to 

ithe sun, and a straight line passing from the sun through 
Fig. 90. 

the eye of the spectator, passes also through the center 
of the bow. When the sun is setting, so that this line 
becomes horizontal, the summit of the bow reaches an 
altitude of about 42, and the bow is then a semicircle. 
When the sun is 42 high, the same line would pass 
42 below the opposite horizon, and the summit of the 
bow would barely reach the horizon. When the sun 
is between these two altitudes, the bow rises as the 
sun descends, composing a larger and larger part of 
a circle, until, as the sun sets, it becomes an entire 

172. The varied colors that adorn the face of nature, 
as seen in the morning and evening cloud, in the tints of 
flowers, in the plumage of birds and wings of certain 
insects, and in the splendid hues of the precious gems, 
arise from the different qualities of different bodies 
in regard to the power of refracting or of reflecting 
light. When a substance reflects all the prismatic 
rays in due proportion, its color is white ; when it 
absorbs them all, its color is black ; and its color is blue, 

does the top of the bow reach when the sun is setting 1 "Where ia 
it when the sun is 42 above the western horizon 1 When the sun 
is between these two points 1 


green, or yellow, when it happens to reflect one of these 
colors, and to absorb all the others of the spectrum. 
These hues are endlessly varied by the power natural i 
bodies have of reflecting a mixture of some of the^ 
primary colors to the exclusion of others, every new- 
proportion producing a different shade. 

SEC. 3. Of Vision. 

173. Whenever we admit into a dark room through : 
an opening in the shutter, light reflected from various*- 
objects without, an inverted picture of these objects? 
will be formed on the opposite wall. A room fitted for- 
exhibiting such a picture, is called a Camera Olscura* 
In a tower which has a window opening toward the 
east, upon a beautiful public square, containing 
churches and other public buildings, and numerous 
trees, and the various objects of a populous city, a t 
little dark chamber is fitted up for a camera obscura, 
having a white concave stuccoed wall opposite to the 
window, ten feet from it, and all the other parts of the 
room painted black. The afternoon, when the sun is;< 
shining bright in the west, and all objects seen to the 
east present their enlightened sides toward the window, 
is the time for forming the picture. For this purpose, 
a round hole about three inches in diameter, is prepared 
in the shutter, which admits the only light that can| 
enter the room. The room is made black every- 
where except the wall that is to receive the picture/ 
otherwise light would be reflected from different parts 
of the room upon the picture ; whereas it is essential 
to its distinctness, that the image should be unac- 

172. How are the colors of natural objects produced 1 When is 
the color white, or red, or blue 1 How are the colors varied 1 

173. How may a picture be formed in a dark room 1 What is it 
called 1 Describe the Camera Obscura mentioned. W nen is the 
time for forming the picture 1 Why is the room painted black, 
except the wall opposite the window 1 

OPTICS. 169 

rjompanied by light from any other source. The wall 
[hat is to receive the picture is made concave, so that 
bvery part of it may be equally distant from the orifice 
in the shutter. 

j 174. We now close the shutter, and instantly there 
Appears on the opposite wall a large picture, repre- 
senting all the varied objects of the landscape seen 
:>om the window, as churches, houses, trees, men and 
ivomen, carriages and horses, and in short every thing 
;hat is in view of the window, including the blue sky, 
ind a few white clouds that are sailing through it. 
Sach is represented in its proportionate size and color, 
Smd if it is moving, in its true motion. Two circum- 
stances, only, impair the beauty of the picture ; one 
s, that it is not perfectly distinct, the other, that it is 
inverted the trees appear to grow downward, and the 
j>eople to walk with their feet above their heads. The 
hicture appears indistinct, because the opening in the 
shutter is so large that rays coming from different ob- 
jects fall upon the picture and mix together, whereas 
?ach point in the image must be formed alone of rays 
poming from a corresponding point in the object. We 
ivill therefore diminish the size of the opening by cov- 
ering it with a slide containing several holes of differ- 
ent sizes. We will first reduce the diameter to an 
inch. The picture is now much more distinct, but yet 
ftot perfectly well defined. We will therefore move 
the slide, and reduce the opening to half an inch. 
Now the objects are perfectly well defined, for through 
5o small an openjng none but the central ray, or axis, 
pf each pencil can enter, and each axis will strike the 
opposite wall in a point distinct from all the rest. But 
though the picture is no longer confused, yet it lacks 
brightness, for so few rays scattered over so large a 

174. On closing the shutter, what appearances present them- 
selves 1 What two circumstances impair the beauty of the picture 1 
Why indistinct How rendered more distinct Why well defined 



Fig. 91. 

surface, are insufficient to form a bright image. Wo 
will now remove the slide, open the original orifice of 
three inches, which lets in a great abundance of light, 
and we will place immediately before the orifice, 
(within the room,) a convex lens of ten feet focus, 
which will collect all the scattered rays into separate 
foci, and thus form a picture at once distinct and bright, 
so that the most delicate objects without, as the trembling 
of the leaves of the trees, and the minutest motions of 
animals, are all very plainly discernible. Only one 
thing is wanting to make the picture perfect, and that 
is, to turn it right side upward. This may be done, 
and is done in some forms of the camera obscura ; but 
for our present purpose, which is to illustrate the prin- 
ciples of the eye, where the image formed is also in- 
verted, it is better as it is. 

175. The eye is a ca- 
mera obscura, and the 
analogy between its prin- 
cipal parts and the contri- 
vances employed to form 
a picture of external ob- 
jects, as in the foregoing 
dark chamber, will appear 
very striking on compari- 
son. Figure 91 represents 
the human eye, which is a 
circular chamber, colored 
black on all sides except the back part, called the re- 
tina, which is a delicate white membrane, like the 
finest gauze, spread to receive the image. The front 
part of the eye, A, is a lens of a shape exactly adapted 
to the purpose it is intended to serve, which projects 

when the orifice is small What does the picture now lack 1 How 
to make it at once well defined and bright 1 

175. Analogy of the eye to the Camera Obscura. Describe ..the 
eye from Fig. 91. 

OPTICS. 171 

forward so as to receive the light that comes in side- 
wise, and guides it into the eye. The pupil is an 
opening between c and c, like the opening in the win- 
dow shutter, just behind which is a convex lens, B, 
which collects all the scattered rays, and brings each 
pencil to a separate focus, where they unite in forming 
a bright and beautifully distinct image of all external 
objects. O represents the optic nerve, by which the 
sensations made on the retina are conveyed to the brain. 
The substances with which the several parts of the eye, 
A, B, and C, are filled, are limpid and transparent, and 
purer than the clearest crystal. 

176. It is essential to distinct vision, that the rays 
which enter the eye should be brought accurately to 
a focus at the place of the retina ; and in ninety-nine 
cases out of a hundred, this adjustment is perfect. But 
in a few instances, the lens, B, called the crystalline 
humor, is too convex, and then the image is formed be- 
fore it reaches the retina. This is the case with near- 
sighted people. Their eyes are too convex ; but by 
wearing a pair of concave spectacles, they can destroy 
the excess of convexity in the eye, and then the crys- 
talline lens will bring the light to a focus on the retina 
and the sight will be distinct. Sometimes, particularly 
as old age advances, the crystalline lens becomes less 
convex, and does not bring the, rays to a focus soon 
enough, but they meet the retina before they have come 
accurately to a focus, and form a confused image. In 
this case a pair of convex spectacles aids the crystal- 
line lens, and both together cause the image to fall ex- 
actly on the retina. As a piece of mechanism, the eye 
is unequalled for its beauty and perfection, and no part 
of the creation proclaims more distinctly both the ex- 
istence and the wisdom of the Creator. 

176. What is essential to distinct vision 1 Imperfection when the 
crystalline lens is too convex how remedied also when not con- 
vex enough remedy 1 Perfection of the eye. 


SEC. 4. Of the Microscope. 

177. The Microscope is an optical instrument, de- 
signed to aid the eye in the inspection of minute objects. 
The simplest microscope is a convex lens, like a spec- 
tacle glass. This, when applied to small objects, as 
the letters of a book, renders them both larger and more 
distinct. When an object is brought nearer and near- 
er to the eye, we finally reach a point within which 
vision begins to grow imperfect. That point is called 
the limit of distinct vision. Its distance is about five 
inches. If the object be brought nearer than this dis- 
tance, the rays come to the eye too diverging for the 
lenses of the eye to bring them to a focus soon enough, 
so as to make their image fall exactly on the retina. 
Moreover, the rays which proceed from the extreme 
parts of the object, meet the eye too obliquely to be 
brought to the same focus with those rays which meet 
it more directly, and hence contribute only to confuse 
the picture. 

178. We may verify these remarks by bringing ' 
gradually toward the eye a printed page with small ! 
letters. When the letters are within two or three ; 
inches of the eye, they are blended together and noth- j 
ing is seen distinctly. If we now make a pin-hole - 
through a piece of paper, and look at the same letters ; 
through this, we find them rendered far more distinct 
than before at near distances, and larger than ordina- 
ry. Their greater distinctness is owing to the exclu- 
sion of those oblique rays which, not being brought by 
the eye to the same focus with the central rays, only 
tend to confuse the image formed by the latter. As 

177. Define the microscope. What is the simplest form of the 
microscope 1 Its effect upon the letters of a book. What is the 
limit of distinct vision 1 Why do objects appear indistinct when 
nearer than this 1 

178. Example in a printed page Appearance through a pin-hole. 
To what is the greater distinctness owing'! The increased brightness] 

OPTICS. 173 

only the central rays of each pencil can enter so small 
an orifice, the picture is made up chiefly of the axes 
of all the pencils. These occupy each a separate 
point in the image, a point where no other rays can 
reach. The increased magnitude of the letters is owing 
to their being seen nearer than ordinary, and thus un- 
der a greater angle, and of course magnified. 

179. A convex lens acts much on the same princi- 
ples, but is still more effectual. It does not exclude 
the oblique rays, but it diminishes their obliquity so 
much as to enable the eye to bring them to a focus, 
at the distance of the retina, and thus makes them con- 
tribute to the brightness of the picture. The object is 
magnified, as before, because it is seen nearer, and 
consequently under a larger angle, so that the eye can 
distinctly recognise minute portions of the object, 
which were before invisible, because they did not oc- 
cupy a sufficient space on the retina. Lenses have 
greater magnifying power in proportion as the convex- 
ity is greater, and of course the focal distance less. 
Since the magnifying power of the microscope arises 
, from its enabling us to see objects nearer and under a 
larger angle, that power is increased in proportion as 
the focal distance is less than the limit of distinct vis- 
ion. The latter being five inches, a lens which has 
a focal distance of one inch, by enabling us to see the 
object five times nearer, enlarges its length and breadth 
each five times, and its surface twenty-five times. 
Lenses have been made capable of affording a distinct 
image of very minute objects, when their focal dis- 
tances were only one-sixtieth of an inch. In this case, 
the magnifying power would be as one-sixtieth to five ; 

179. Explain the mode in which a convex lens acts. Why it 
makes objects appear brighter and larger. What lenses have the 
greatest magnifying power 1 Power of a lens of one inch focus 
of one sixtieth of an inch focus. 



or it would magnify the length and breadth each 300 
times, and the surface 90,000 times. 

180. The Magic Lantern and Solar Microscope owe 
their astonishing effects to the magnifying power of a 
simple lens. When the image so much exceeds the 
object in magnitude, were the object only enlightened 
by the common light of day, when it came to be diffu- 
sed over so great a space, it would be very feeble, and 
the image would be obscure and perhaps invisible. 
The two instruments just named, have each an appa- 
ratus connected with the magnifying lens, which serves 
to illuminate the object highly, so that when the rays 
that proceed from it and form the enlarged image are 
spread over so great a space, they may still be suffi- 
cient to render the image bright and distinctly visible. 
Fig. 92. 

181. In the Magic Lantern, the illumination is af- 
forded by a lamp, "the light of which is reflected from 
a concave mirror placed behind it, which makes the 
light on that side return to unite with the direct light 

180. To what are the effects of the Magic Lantern and the Solar 
Microscope owing 1 Use of all the other parts of the apparatus, ex- 
cept the magnifier 1 

181. How is the illumination effected in the magic lantern 1 De- 
scribe Fig. 92. What sorts of objects are exhibited 1 

OPTICS. 175 

of the lamp, so that both fall on a large lens which col- 
lects them upon the object, thus strongly illuminating 
it. The foregoing diagram exhibits such a lantern, 
where the concave mirror behind is seen to reflect 
back the light to unite with that which proceeds di- 
rectly from the lamp, so that both fall on the large con- 
vex lens at C, which collects them upon the object at 
B. This is usually painted in transparent colors on 
glass, and may be a likeness of some individual, small 
in the picture, but when magnified by the lens, A, and 
the image thrown on a screen or wall, F, will appear 
as large as life, and in strong colors ; or the objects 
may be views of the heavenly bodies, which are thus 
often rendered very striking and interesting ; or they 
may illustrate some department of natural history, as 
birds, fishes, or plants. 

182. The Solar Microscope is the same in princi- 
ple with the Magic Lantern, but the light of the sun 
instead of that of a lamp is employed to illuminate the 
object. As a powerful light may thus be commanded, 
very great magnifiers can be employed ; for if the ob- 
ject is highly illuminated, the image will not be feeble 
or obscure when spread over a great space. By means 
of this instrument, the eels in vinegar, which are usu- 
ally so small as to be invisible to the naked eye, may 
be made to appear six feet in length, and, as their mo- 
tions as well as dimensions are magnified, they will 
appear to dart about with surprising velocity. The 
finest works of art, when exhibited in this instrument, 
appear exceedingly coarse and imperfect. The eye 
of a finished cambric needle appears full of rough pro- 
jections ; the blade of a razor looks like a saw ; and 
the finest muslin exhibits threads as large as the cable 
of a ship. Thus, the small and almost invisible insect 

182. Solar Microscope, how it differs from the Magic Lantern 
Why greater magnifiers can be used appearance of the eels in 
vinegar. How do the works of art appear *? How small insects 1 



represented in figure 93, gives out, when illuminated, 
so few rays, that when spread over the large surface 
of the image, the light would be too feeble to render 
the image visible ; but, on strongly illuminating the 

Fig. 93. 

insect by concentrating upon it a large beam of the 
sun's light, the image becomes distinct and beautiful, 
although perhaps a million times as large as the object. 
Even the minute parts of the insect, as the hairs on the i 
legs, are revealed to us by the microscope. 

SEC. 5. Of the Telescope. 

183. The Telescope is an instrument employed for 
viewing distant objects. It aids the eye in two ways ; . 
first, by enlarging the angle under which objects are ; 
seen, and, secondly, by collecting and conveying to : 
the eye a much larger amount of the light that pro- 
ceeds from the object, than would enter the naked pu- 
pil. We first form an image of a distant object, the 

183. Define the Telescope. In what two ways does it aid the eye 1 

OPTICS. 177 

moon, for example, and then magnify that image Try a 
microscope. The image may be formed either by a 
concave mirror or a convex lens, for both, as we have 
seen, form images. Although we cannot go to distant 
objects, as the moon and planets, so as to view them 
under the enlarged dimensions in which they would 
then appear, yet by applying a microscope to the image 
of one of those bodies, we may make it appear as it 
would do were we to come much nearer to it. To ap- 
ply a microscope which magnifies a hundred times, is 
the same thing as to approach a hundred times nearer 
to the body. 

Fig. 94. 

184. Let A B C D represent the tube of the tele- 
scope. At the front end, or the end which is directed 
toward the object, (which we will suppose to be the 
moon,) is inserted a convex lens, L, which receives the 
rays of light from the moon, and collects them into the 
focus at a, forming an image of the moon. This 
image is viewed by a magnifier attached to the end, 
B C. The lens, L, is called the object-glass, and the 
microscope, in B C, the eye-glass. A few rays of light 
only from a distant object, as a star, can enter so small 

State the main principle of this instrument. How may the image 
be formed 1 How it brings objects nearer to us. 

184. Describe the telescope as represented in Fig. 94. Point out 
the object-glass, and the eye-glass. What is the use of a large ob- 


a space as the pupil of the eye : but a lens one foot in 
diameter will collect a beam of light equal to a cylin- 
der of the same dimensions, and convey it to the eye. 
The object-glass merely forms an image of the object, 
but does not magnify ; the microscope or eye-glass 
magnifies. By these means, many obscure celestial 
objects become distinctly visible, which would other- 
wise be too minute, or not sufficiently luminous, to be 
seen by us. A telescope like the foregoing, having 
simply an object-glass and an eye-glass, inverts ob- 
jects, since the rays cross each other before they form j 
the image. By employing more lenses, it may be I 
turned back again, so as to appear in its natural posi-, 
tion, as is usually done in spy-glasses, or the smaller , 
telescopes used in the daytime. But since every lens/ 
absorbs and extinguishes a certain portion of the light, 
and since, in viewing the heavenly bodies, we usually 
wish to save as much of the light as possible, astro- 
nomical telescopes are constructed with these two 
glasses only. 

185. Instead of the convex object-glass, we may 
employ the concave mirror to form the image. When 
the lens is used, the instrument is called a refracting 
telescope ; when a concave mirror is used, it is called 
a reflecting telescope. Large reflectors are more easily 
made than large refractors, since a concave mirror 
may be made of any size ; whereas, it is very difficult 
to obtain glass that is sufficiently pure for this purpose 
above a few inches in diameter, although Refractors 
are more perfect instruments than Reflectors, in pro- 
portion to their size. Sir William Herschel, a great 
astronomer of England, of the last century, made a 

ject-glass 1 "Which glass collects the light which magnifies 1 Can 
the image be made "to appear erect 1 "Why not done in the astro- 
nomical telescope 1 

185. Point out the distinction between refracting and reflecting 
telescopes. Give an account of Herschel's great telescope. 

OPTICS. 179 

reflecting telescope forty feet in length, with a concave 
mirror more than four feet in diameter. The mirror 
alone weighed nearly a ton. So large and heavy an 
instrument must require a vast deal of machinery to 
work it and keep it steady ; and accordingly, the frame- 
work surrounding it was formed of heavy timbers, and 
resembled the frame of a house. When one of the 
largest of the fixed stars, as Sirius, was entering the 
field of this telescope, its approach was announced by 
a bright dawn, like that which precedes the rising sun; 
and when the star itself entered the field, the light was 
too dazzling to be seen without a colored glass to pro- 
tect the eye. 

The telescope has made us acquainted with innu- 
merable worlds, many of which are fitted up in a style 
of far greater magnificence than our own. To the in- 
teresting and ennobling study of these, let us next di- 
rect our attention. 








186. ASTRONOMY is that science which treats of th 
heavenly bodies. More particularly, its object is t 
teach what is known respecting the Sun, Moon, Planets 
Comets, and Fixed Stars ; and also to explain th 
methods by which this knowledge is acquired. 

187. Astronomy is the oldest science in the world 
but it was cultivated among the ancients chiefly for th 
purposes of Astrology. Astrology was the art of fore 
telling future events ly the stars. Its disciples pro 
fessed especially to be able to tell from the appearance 
of the stars at the time of any one's birth, what woulc 
be his course and destiny through life ; and, respecting 
any country, and public events, what would be 
their fate, what revolutions they would undergo, wha 
wars and other calamities they would suffer, or wha 
good fortune they would experience. Visionary as 

186. Define Astronomy "What is its object* 

187 Antiquity of the scienceFor what purpose vfos it cultivate* 


this art was, it nevertheless led to the careful obser- 
vation and study of the heavenly bodies, and thus laid 
the foundations of the beautiful temple of modern as- 

188. Astronomy is a delightful and interesting study, 
when clearly understood ; but it is very necessary to a 
clear understanding of it, that the learner should think 
for himself, and labor to form an idea in his mind of 
the exact meaning of all the circles, lines, and points 
of the sphere, as they are successively defined ; and 
if any thing at first appears obscure, he may be assured 
that by patient thought it will clear up and become 
easy, and then he will understand the great machinery 
of the heavens as easily as he does that of a clock. 
" Patient thought," was the motto of Sir Isaac New- 
ton, the greatest astronomer that ever lived ; and no 
other way has yet been discovered of obtaining a clear 
knowledge of this sublime science. 

189. Let us imagine ourselves standing on a huge 
ball, (for such is the earth,) in a clear evening. Al- 
though the earth is large, compared with man and his 
works, yet it is very small, compared with the vast 
extent of the space in which the heavenly bodies move. 
When we look upward and around us at the starry 
heavens, we must conceiv9 of ourselves as standing 
on a small ball, which is encircled by the stars on all 
sides of it alike, as is represented over the leaf; and 
we must consider ourselves as bound to the earth by 
an invisible force, (gravity,) as truly as though we 
were lashed to it with cords. We are, therefore, in 
no more danger of falling off, than needles are of fall- 

amongthe ancients'? Define astrology. What did its disciples pro- 
fess 1 To what good did it lead * 

188. What is necessary to a clear understanding of Astronomy 1 
What was the motto of Newton 1 

189. Where shall we imagine ourselves standing 1 What is said 
of the size of the earth 1 Are persons on the opposite side of the 




ing from a magnet or loadstone, when they are attached 
to it on all sides. We must thus familiarize ourselves 
to the idea that up and down are not absolute directions 

in space, but we must endeavor to make it seem to us 
up in all directions from the center of the earth, and 
down on all sides toward the center. If people on the 
opposite side of the globe seem to us to have their 
heads downward, we seem to them to have ours in the 
same position ; and, twelve hours hence, we shall be 
in their situation and they in ours. We see but half j 
the heavens at once, because the earth hides the other 
part from us ; but if we imagine the earth to grow less 
and less until it dwindles to a point, so as not to ob- 
struct our view in any direction, then we should see 
ourselves standing in the middle of a vast starry sphere, 
encompassing us alike on all sides. It is such a view 

earth in danger of falling offl What idea must we form of up and 
down ? How should we view the heavens if the earth were so 
small as not to obstruct our view "? 


of the heavens that the astronomer has continually in 
the eye of his mind. 

190. We are apt to bring along with us the first 
impressions of childhood ; namely, that the sun, moon, 
and stars, are all fixed on the surface of the sky, which 
we imagine to be a real surface, like that of an arched 
ceiling ; but it is time now to dismiss such childish no- 
tions, and to raise our thoughts to more just views of 
the creation. Our eyesight is so limited that we can- 
not distinguish between different- distances, except 
for a moderate extent ; beyond, all objects seem to us 
at the same distance, whether they are a hundred or a 
million miles off. The termination of this extent of 
our vision being at equal distances on all sides of us, 
we appear to stand under a vast dome, which we call 
the sky. The azure color of the sky, when clear, is 
nothing else than that of the atmosphere itself, which, 
though colorless when seen in a small volume, betrays 
a hue peculiar to itself when seen through its whole 
extent. Were it not for the atmosphere, the sky would 
appear black, and the stars would seem to be so many 
gems set in a black ground. 

191. For the purpose of determining the relative 
situation of places, both on the earth and in the heav- 
ens, the various circles of the sphere are devised ; 
but before contemplating the sphere marked up as ar- 
tificial representations of it are, we must think of our- 
selves as standing on the earth, as on a point in the 
midst of boundless space, and see, with our mental eye, 
the pure sphere of the heavens, undefaced with any 
such rude lines. If we could place ourselves on any 

190. What erroneous conceptions are we apt to form in childhood 
of the sun, moon, and stars * Impossibility of distinguishing differ- 
ent distances by the eye. Under what do we appear to stand 1 To 
what is the blue color of the sky owing 1 

191. For what purpose are the circles of the sphere devised 1. 
What must we do before studying the artificial representations of 
the sphere 1 If we could stand on one of the stars, what should we 



. 96. 

one of the stars, we should see a starry firmament over 
our heads, similar to that we see now. But although 
we obtain the most correct and agreeable, as well as : 
the most sublime views of the heavenly bodies, when ;; 
we think of them as they are in nature bodies scat- \\ 
tered at great distances from each other, through bound. | 
less space yet we cannot make much progress in the ; 
science of astronomy, unless we learn the artificial di- 
visions of the sphere. Let us, therefore, now turn our 
attention to these. 

192. The definitions of the different lines, points, I 
and circles, which are used in astronomy, and the j 
propositions founded on them, compose the doctrine of 
the sphere. Before these definitions are given, let us 
attend to a few particulars respecting the method of 
measuring angles. (See Fig. 96.) A line drawn 

from the center to the 
circumference of a cir- 
cle, is called a radius, as 
C D, C B, or C K. Any 
part of the circumfer- 
ence of a circle is called 
an arc, as A B, or B D. 
An angle is measured by 
an arc, included between 
two radii. Thus, in fig- 
ure 96, the angle in- 
cluded between the two 
radii, C A and C B, that 
is, the angle A C B, is 
measured by the arc A B. Every circle is divided 
into 360 equal parts, called degrees ; and any arc, as 
A B, contains a certain number of degrees, according 

see 1 When do we obtain the most agreeable and sublime views of 
the heavenly bodies'? What else is necessary to our progress 1 

192. Define the doctrine of the sphere. What is the radius of a cir 
cle an arc an angle 1 Explain by Fig. 96. Into how many de- 


its length. Thus, if an arc, A B, contains 40 
degrees, then the opposite angle is said to be an angle 
of 40 degrees, and to be measured by A B. But this 
arc is the same part of the smaller circle that E F is 
of the greater. The arc A B, therefore, contains the 
same number of degrees as the ipirc E F, and either 
may be taken as the measure of the angle A C B. As 
the whole circle contains 360 degrees, it is evident that 
the quarter of a circle, or quadrant, contains 90 degrees, 
and that the semi-circle contains 180 degrees. 

193. A section of a sphere, cut through in any 
direction, is a circle. Great circles are those which 
pass through the center of a sphere, and divide it into 
two equal hemispheres. Small circles are such as do 

not pass through the center, but divide the sphere into 
two unequal parts. This distinction may be easily 
exemplified by cutting an apple first through the center, 
and then through any other part.* The first section 
will be a great, and the second a small circle. The 

' axis of a circle is a straight line passing through its 
center at right angles to its plane. If you cut a circle 
out of pasteboard, and thrust a needle through the center, 
perpendicularly, it will represent the axis of the circle. 
The pole of a great circle, is the point on the sphere 
where its axis cuts through the sphere. Every great 
circle has two poles, each of which is everywhere 90 
degrees from that circle. All great circles of the sphere 
cut each other in two points, diametrically opposite, and 
consequently their points of section are 180 degrees 

* It is strongly recommended that young learners be taught to verify the de- 
finitions in the manner here proposed. 

grees is every circle divided 1 Does the arc of a small circle contain 
the same number of degrees as the corresppnding arc of a large 
circle 1 How many degrees in a quadrant in a semi-circle 1 

193. What figure does any section of a sphere produce 1 Define 

great circles small circles. How may this distinction be exempli- 

lied 1 Define the axis of a circle the rjole. How many poles has 

every great circle 1 How many degrees is the pole from the circum- 



apart. Thus, if we cut the apple through the center, 
in two different directions, we shall find that the points 
where the circles intersect one another, are directly 
opposite to each other, and hence the distance between 
them is half round the apple, and, of course, 180 de- 
grees. A point on the sphere, 90 degrees distant from 
any great circle, is the pole of that circle ; and every 
circle on the globe, drawn from the pole to the circum- 
ference of any circle, is at right angles to it. Such a 
circle is called a secondary of the circle through whose 
pole it passes. 

194. In order to fix the position of any place, either 
on the surface of the earth or in the heavens, both the 
earth and the heavens are conceived to be divided into 
separate portions, by circles which are imagined to cut 
through them in various ways. The earth, thus 
intersected, is called the terrestrial, and the heavens 
the celestial sphere. The great circles described on 
the earth, extended to meet the concave sphere of the 
heavens, become circles of the celestial sphere. 

The Horizon is the great circle which divides the 
earth into upper and lower hemispheres, and separates 
the visible heavens from the invisible. This is the 
rational horizon : the sensible horizon is a circle touch- 
ing the earth at the place of the spectator, and is 
bounded by the line in which the earth and sky seem 
to meet. The poles of the horizon are the zenith and 
nadir. The zenith is the point directly over our heads; 
the nadir, that directly under our feet. The plumb- 
line, (such as is formed by suspending a bullet by a 
string,) coincides with the axis of the horizon, and 
consequently is directed toward its poles. Every 

ference 1 How does a great circle passing through the pole of another 
great circle cut the circle 1 What is such a circle called 1 

194. How are the earth and heavens conceived to be divided 1 
What is the terrestrial, and what the celestial sphere 1 ' How do 
terrestrial circles become celestial * Define the horizon. Distinguish 
between the rational and the sensible horizon. Define the zenith 


place on the surface of the earth has its own horizon ; 
and the traveller has a new horizon at every step, 
always extending 90 degrees from him in every 

195. Vertical circles are those which pass through 
the poles of the horizon, (the zenith and nadir,) perpen- 
dicular to it. The Meridian is that vertical circle 
which passes through the north and south points. The 
Prime Vertical, is that vertical circle which passes 
through the east and west points. The altitude of a 
heavenly body, is its elevation above the horizon, 
measured on a vertical circle ; the azimuth of a body 
is its distance, measured on the horizon, from the meri- 
dian to a vertical circle passing through that body ; and 
the amplitude of a body is its distance, on the horizon, 
north or south of the prime vertical. 

196. In order to make these definitions intelligible 
and familiar, I invite the young learner, who is anxious 
to acquire clear ideas in astronomy, to accompany me 
some fine evening under the open sky, where we can 
have an unobstructed view in all directions. A ship 
at sea would afford the best view for our purpose, but 
a level plain of great extent will do very well. We 
carry the eye all round the line in which the sky seems 
to rest upon the earth : this is the horizon. I hold a 
line with a bullet suspended, and this shows me the 
true direction of the axis of the horizon ; and I look 
upward in the direction of this line to the zenith, 
directly over my head, and downward toward the 
| nadir. If I mark the position of a star exactly in the 

zenith, as indicated by the position of the plumb-line, 

and nadir. Toward what points is the plumb line directed 1 How 
many horizons can be imagined 1 

195. Define vertical circles the meridian the prime vertical 
' altitude azimuth amplitude. 

196. What is proposed in order to make these definitions intelli- 

e'.ble and iamiliarl What situation would afford the best view 1 
escribe how we shall successively denote the position of the axis 01 


and then turn round and look upward toward the zenith, j, 
I shall probably not see the star, because I do not look; 
high enough. Most people will find, if they first fix* 
upon a star as being in the zenith when their faces are 
toward the south, and then turning round to the north, 
fix upon another star as near the zenith, (without! 
reference to the first,) they will find that the two stars, 
are several degrees apart, the true zenith being half 
way between them. This arises from the difficulty of 
looking directly upward. 

197. Having fixed upon the position of the zenith, Ii 
will point my finger to it, and carry the finger down to: 
the horizon, repeating the operation a number of times,; 
from the zenith to different points of the horizon : the; 
arcs which my finger may be conceived to trace oul 
on the face of the sky, are "arcs of vertical circles. IJ 
will now direct my finger toward the north point ow 
the horizon, (having previously ascertained its position.; 
by a compass,) and carry it upward through the| 
zenith, and down to the south point of the horizon : 
this is the meridian. From the south point, I carry 4 
my finger along the horizon, first toward the east, 
and then toward the west, and I measure off arcs of 
azimuth. I might do the same from the north point, 
for azimuth is reckoned east and west from either the* 
north or the south point. I will again direct my, 
finger to the western point of the horizon, and carry, 
it upward through the zenith to the east point, and 
I shall trace out the prime vertical. From this, 
either on the eastern or the western side, if Ii 
carry my finger along the horizon, north and south, 
I shall trace out arcs of amplitude. I will finally fix 

the horizon the zenith and nadir. Difficulty of looking directly to 
the zenith. 

197. How to mark out with the ringer vertical circles the meri- 
dian arcs of azimuth the prime vertical arcs of amplitude arcs 
of altitude 1 


m y eve on a certain bright star, and try to determine 
hoV far it is above the horizon. This will be its alti- 
I tilde. It appears to be about one third of the way 
'from the horizon to the zenith ; then its altitude is 30 
| degrees. But we are apt to estimate the number of 
! degrees near the horizon too large, and near the zenith 
(too small, and therefore I look again more attentively, 
: making some allowance for this source of error, and 
I judge the altitude of the star to be about 27 degrees, 
and of course its zenith distance 63 degrees. 

19&. The Axis of the earth, is the diameter on which 
the earth is conceived to turn in its daily revolution 
ifrom west to east. The same line continued until it 
| meets the concave of the heavens, constitutes the axis 
I of the celestial sphere. We will take a large round 
; apple, and run a knitting-needle through it in the di- 
jrection of the eye and stem. The part of this that 
'lies within the apple, represents the axis of the earth, 
:and its prolongation (conceived to be continued to the 
]sky,) the axis of the heavens. We do not suppose 
that there is any such actual line on which the earth 
1 turns, any more than there is in a top on which it 
spins ; but it is nevertheless convenient to imagine 
^such a line, and to represent it by a wire.* The poles 
of the earth are the extremities of the earth's axis; 
the poles of the heavens are the extremities of the ce- 
lestial axis. 

199. The Equator is a great circle, cutting the axis 

of the earth at right angles. The intersection of the 

\ plane of the equator with the surface of the earth, con- 

1 * Experience shows that it is necessary to guard young learners from the 
j error of supposing that our artificial representations of the sphere actually 
represent things as they are in nature. 

193. Define the axis of the earth axis of the celestial sphere. 
'Ho\v are both represented by means of an apple 7 Is there any 
'such actual line on which the earth turns 1 Distinguish between the 
i poles of the earth and the poles of the heavens. 


stitutes the terrestrial, and its intersection with the 
concave sphere of the heavens, the celest.ial equator. 
We have before seen (Art. 195) that every place on ther 
earth has its own horizon. Wherever one stands on i 
the earth, he seems to be in the center of a circle 
which bounds his view. If he is at the equator, this 
circle passes through both the poles ; or, in other- 
words, at the equator the poles lie in the horizon. . 
Let us imagine ourselves standing there on the 21sti 
of March, when the sun rises due east and sets due| 
west, and appears to move all day in the celestial! 
equator, and let us think how it would seem to see theft 1 
sun, at noon, directly over our heads, and at night tdf 
see the north star just glimmering on the north poinfii 
of the horizon. If we sail northward from the equa-' 
tor, the north star rises just as many degrees above the| 
horizon as we depart from the equator ; so that by the 
time we reach the part of the globe where we live! 
the north star has risen almost half way to the zenith," 
and the axis of the sphere which points toward theo 
north star, seems to have changed its place as we have- 1 
changed ours, and to have risen up so as to make a i 
large angle with the horizon, and the sun no longer \ 
mounts to the zenith at noon. 

200. Now it is not the earth that has shifted its po4 
sition ; this constantly maintains the same place, and 
so does the equator and the earth's axis. Our horizon 
it is that has changed ; as we left the equator, a new 
horizon succeeded at every step, reaching constantly.' 
farther and farther beyond the pole of the earth, on 
dipping .constantly more and more below the celestial 
pole ; but being insensible of this change in our hori- 

199. Define the equator. Distinction between the terrestrial and 
the celestial equator. Where do the poles of the equator lie 1 How * 
would the sun appear to move to a spectator on the equator 1 
Where would the north star appear 1 How, when we sail northward 
from the equator 1 What apparent change in the earth's axis 7 

200. What has caused these changes 1 It' we sail quite to the north 


zon, the pole it is that seems to rise, and if we were 
] to sail quite to the north pole of the earth, the north 
star would be directly over our heads, and the equator 
would have sunk quite down to the horizon ; and now 
;the sun, instead of mounting up to the zenith at noon, 
just skims along the horizon all day ; and, at night, at 
seasons of the year when the sun is south of the equator, 
all the stars appear to revolve in circles parallel to the 
horizon, the circles of revolution continually growing 
less as we look higher and higher, until those stars 
which are near the zenith scarcely appear to revolve 
!at all. Those who sail from the equator toward the 
:pole, and see the apparent paths of the sun and stars 
change so much, can hardly help believing that those 
! bodies have been changing their courses; but all these 
I appearances arise merely from the spectator's chang- 
jing his own horizon, that is, constantly having new 
lones, which cut the axis of the earth at different an- 

201. The Latitude of a place on the earth, is its dis- 
tance from the equator, north or south. The Longi- 
tude of a place is its distance from some standard 
meridian, east or west. The meridian usually taken 

^as the standard, is that of the observatory of Green- 
wich, near London; and when we say that the longi- 
j tude of New York is 74 degrees, we mean that the 
\ meridian of New York cuts the equator 74 degrees 
west of the point where the meridian of Greenwich 
cuts it. 

202. The Ecliptic is the great circle in which the 
^ earth performs its annual revolution around the sun. 
; It passes through the center of the earth and the cen- 
ter of the sun. It is found, by observation, that the 
earth does not lie with its axis perpendicular to the 

i pole, where will the north star appear'? Where the equator 1 How 
j would the sun and stars appear to revolve in their daily progress 1 

201. Define the latitude of a place on the earth the longitude 
i from what place is it reckoned 1 


plane of the ecliptic, so as to make the equator coin-| 
cide with it, but that it is turned about 23 degreed 
out of a perpendicular direction, making an angle with? 
the plane itself of 68| degrees. The equator, there.] 
fore, must be turned the same distance out of a coin- 
cidence with the ecliptic, the two circles making an 
angle with each other of 23J degrees. The Equinoc* 
Hal Points, or Equinoxes, are the points where the 
ecliptic and equator cross each other. The time when 
the sun crosses the equator in going northward, is< 
called the vernal, and in returning southward, the aw*> 
tumnal equinox. The vernal equinox occurs about! 
the 21st of March, and the autumal about the 22d off 
September. The Solstitial Points are the two point** 
of the ecliptic most distant from the equator. ThJ 
times when the sun comes to them are called the SolA 
stices. The summer solstice occurs about the 22<M 
of June, and the winter solstice about the 22d of De4 

203. The ecliptic is divided into twelve equal parts, 
of 30 degrees each, called Signs, which, beginning at 
the vernal equinox, succeed each other in the follow-' 
ing order, being each distinguished by characters or'i 
symbols, by which the student should be able to re-| 
cognise the signs to which they severally belong whence- 
ever he meets with them. 

1. Aries, 


7. Libra, ^ 

2. Taurus, 

8. Scorpio, 1H. 

3. Gemini, 


9. Sagittarius, 1 

4. Cancer, 


10. Capricornus, Y? 

5. Leo, 


11. Aquarius, 2 

6. Virgo, 


12. Pisces, > 

202. Define the ecliptic. What is the angle of inclination of the 
ecliptic to the equator'? What are the equinoctial points or equi- 
noxes the vernal equinox the autumnal the solstitial points 
the solstices 1 When do they occur 1 

203. Iio\v is the ecliptic divided 1 Name the signs of the zodiac 
and recognise each by its character. 


204. The position of a heavenly body is referred to 
by its right ascension and declination, as in Geography 
we determine the situation of places by their latitudes 
and longitudes. Right Ascension is the angular dis- 
tance from the vernal equinox, reckoned on the celes- 
tial equator, as we reckon longitude on the terrestrial 
equator from Greenwich. Declination is the distance 
of a body from the celestial equator, either north or 
south, as latitude is counted from the terrestrial equa- 
tor. Celestial Longitude is reckoned on the ecliptic 
from the vernal equinox, and celestial Latitude from 
the ecliptic, north or south. 

205. Parallels of Latitude are small circles parallel 
jto the equator. They constantly diminish in size, as 
we go from the equator to the pole. The Tropics are 
the parallels of latitude which pass through the sol- 
Istices. The northern tropic is called the tropic of Can- 
cer ; the southern, the tropic of Capricorn. The Po- 
lar Circles are the parallels of latitude that pass through 
the poles of the ecliptic, 23J degrees from the poles of 
the earth. That portion of the earth which lies be- 
tween the tropics, on either side of the equator, is called 
the Torrid Zone ; that between the tropics and the 
-polar circles, the Temperate Zone ; and that between 
the polar circles and the poles, the Frigid Zone. The 
Zodiac is the part of the celestial sphere which lies 
about eight degrees on each side of the ecliptic. This 
portion of the heavens is thus marked off by itself be- 
cause the paths of the planets are confined to it. 

206. After having endeavored to form the best idea 
,we can of the circles, and of the foregoing definitions 
[relating to the sphere, we shall derive much aid from 

204. Define right ascension declination celestial longitude 
celestial latitude. 

205. Parallels of latitude how do they change as we go from 
the equator'? The tropics polar circles torrid zone -temperate 
^one frigid zones zodiac. 



inspecting an artificial globe, and seeing how these [ 
various particulars are represented there. But every i 
learner, however young, can adopt, with great advan- 1 
tage, the following easy device for himself. To repre- \ 
sent the earth, select a large apple, (a melon, when in-'; 
season, will be found still better.) The eye and the 
stem of the apple will indicate the position of the two j 
poles of the earth. Applying the thumb and finger | 
of the left hand to the poles, and holding the apple so I 
that the poles may be in a north and south line, turaj 
this little globe from west to east, and its motion will ; 
correspond to the daily motion of the earth. Pass a 
wire or a knitting-needle through the poles, and it wiltl 
represent the axis of the sphere. A circle cut round \ 
the apple half way between the poles, will be the \ 
equator ; and several other circles cut between the \ 
equator and the poles, parallel to the equator, will re- 
present 'parallels of latitude ; of which two, drawn 23 ! 
degrees from the equator, will be the tropics, and two j 
others, at the same distance from the poles, will be the ! 
polar circles. The space between the tropics, on both j 
sides of the equator, will be the torrid zone ; between 
the tropics and polar circles, the two temperate zones ; 
and between the polar circles and the poles, the twcl 
frigid zones. A great circle cut round the apple, I 
passing through both poles, in a north and south direc- J 
tion, will represent the meridian, and several other \ 
great circles drawn through the poles, and, of course, , 
perpendicularly to the equator, will be secondaries to 
the equator, constituting meridians, or hour circles. A 
great circle, cut through the center of the apple, from 
one tropic to the other, would represent the plane of the 

206. After forming as clear an idea as we can of the divisions of 
the sphere, to what aids shall we resort 1 How shall we represent 
the earth its poles the daily motion axisequator parallels of 
latitude tropics polar circles zones meridians or hour circles 
solstices equinoctial points'? 


J ecliptic, and its intersection with the surface of the 
| apple, would be the terrestrial ecliptic. The points 
where this circle meets the tropics, indicate the position 
of the solstices ; and its intersections with the equator, 
the equinoctial points. 




207. WHEREVER we are situated on the surface of the 
earth, we appear to be in the center of a vast sphere, 
on the concave surface of which all celestial objects 
are inscribed. If we take any two points on the sur- 
face of the sphere, as two stars, for example, and 
imagine straight lines to be drawn from them to the eye, 
the angle. included between these lines will be measured 
by the arc of the sky contained between the two 
Fig. 97. 

points. Thus, if D B H, Fig. 97, represents the con- 

207. How to measure the angular distance between two stars. 
Illustrate by Fig. 97. Why may we measure the angle on the small 
circle G F K 1 


cave surface of the sphere, A, B, two points on it, as 
two stars, and C A, C B, straight lines drawn from the 
spectator to those points, then the angular distance be- 
tween them is measured by the arc A B, or the angle 
A C B. But this angle may be measured on a much 
smaller circle, G F K, since the arc E F will have 
the same number of degrees as the arc A B. 

208. The simplest mode of taking an angle between 
two stars, is by means of an arm opening at the joint 
like the blade of a penknife, the end of the arm moving 
like C E upon the graduated circle G F K. In fact, 
an instrument constructed on this principle, resembling 
a carpenter's rule with a folding joint, with a semicircle 
attached, constituted the first rude apparatus for meas- 
uring the angular distance between two points on the 
celestial sphere. Thus, the sun's elevation above the 
horizon might be ascertained by placing one arm of 
the rule on a level with the horizon, and bringing the 
edge of the other into a line with the sun's centre. 
The common surveyor's compass affords a simple ex- 
ample of angular measurement. Here the needle lies 
in a north and south line, while the circular rim of the 
compass, when the instrument is level, corresponds to 
the horizon. Hence, the compass shows the azimuth 
of an object, or how many degrees it is east or west 
of the meridian. In several astronomical instruments, 
the telescope and graduated circles are united ; the 
telescope enables us to see minute objects or points, 
and the graduated circle enables us to measure angu- 
lar distances from one point to another. The most im- 
portant astronomical instruments are the Telescope, the 
Transit Instrument, the Astronomical Clock, and the 

203. What is the simplest mode of taking the angle between two 
stars 1 Example of angular measurement l>y the surveyor's com- 
pass. What angle or arc does it measure 1 Why do some instru- 
ments unite the telescope with a graduated circle 1 


209. The Telescope has been already described and 
its principles explained, (Art. 184.) We have seen 
that it aids the eye in two ways : first, by collecting 
and conveying to the eye a larger beam of light than 
would otherwise enter it, thus rendering objects more 
distinct, and many visible that would otherwise be in- 
visible for want of sufficient light ; and, secondly, by 
enlarging the angle under which objects are seen, and 
thus bringing distinctly into view such as are invisible, 
or obscure to the naked eye from their minuteness. 
When the telescope is used by itself, it is for obtain- 
ing brighter and more enlarged views of the heavenly 
bodies, especially the moon and planets. With the 
larger kinds of telescopes, we obtain many grand and 
interesting views of the heavens, and see millions of 
worlds revealed to us that are invisible to the naked 

210. The Transit Instrument (Fig. 98, p. 198) is a 
telescope firmly fixed on a stand, so as to keep it per- 
fectly steady, and permanently placed in the meridian. 
The object of it is to determine when bodies cross the 
meridian, or make their transit over it ; or, in other 
words, to show the precise instant when the center of 
a heavenly body is on the meridian. The Astronomi- 
cal Clock is the constant companion of the transit in- 
strument. This clock is so regulated as to keep exact 
pace with the stars, which appear to move round the 
earth from east to west once in twenty-four hours, in 
consequence of the earth turning on its axis in the 
same time from west to east. The time occupied in 
one complete revolution of the earth, (which is indica- 
ted by the interval occupied by a star from the me- 

209. How does the telescope aid the eye 1 When the telescope is 
used by itself, for what purpose is if? What views do we obtain 
with the larger kinds of telescopes! 

210. Define the Transit Instrument. What is the object of it * 
What does it show 1 What instrument accompanies it 1 With 
what does the astronomical clock keep pace 1 What occasions the 




ridian round to the meridian again,) is called a sidereal 
day. It is, as we shall see hereafter, shorter than the 

Fig. 98. 

solar day as measured by the return of the sun to the 
meridian. The astronomical clock is so regulated as 
to measure the progress of a star, indicating an hour 
for every fifteen degrees, and twenty-four hours for the 
whole period of the revolution of a star. Sidereal time 
commences when the vernal equinox is on the meridian, 
just as solar time commences when the sun is on the 

apparent movement of the stars from east to west 1 Define a side-* 
real day. To how many degrees does an hour correspond 1 Wheu 
does sidereal time commence 1 


211. Any thing becomes a measure of time which 
-divides duration equally. The celestial equator, there- 
fore, is precisely adapted to this purpose, since, in the 
-daily revolution of the heavens, equal portions of it 
pass under the meridian in equal times. The only 
difficulty is, to ascertain the amount of these portions 
for given intervals. Now the astronomical clock 
shows us exactly this amount, for, when regulated to 
.sidereal time, the hour hand keeps exact pace with. the 
vernal equinox, revolving once on the dial plate of the 
.clock while the equator turns once by the revolution 
.of the earth. The same is true, also, of all the small 
circles of diurnal revolution : they all turn exactly at 
the same rate as the equator, and a star situated any- 
where between the equator and the pole, will move in 
its diurnal circle along with the clock, in the same 
manner as though it were in the equator. Hence, if 
live note the interval of time between the passage of 
#ny two stars, as shown by the clock, we have a meas- 
ure of the number of degrees by which they are distant 
from each other in right ascension. We see now how 
.easy it is to take arcs of right ascension : the transit 
instrument shows us when a body is on the meridian ; 
the clock indicates how long it is since the vernal equi- 
nox passed it, which is the right ascension itself. (Art. 
204.) It also tells us the difference of right ascension 
between any two bodies, simply by indicating the dif- 
ference in time between their periods of passing the 
meridian. I observed a star pass the central wire of 
the transit instrument (which was exactly in the me- 
ridian) three hours and fifteen minutes of sidereal time ; 
hence, as one hour equals fifteen degrees, three hours 

211. How may any thing become a measure of time 1 Why is the 
celestial equator peculiarly adapted to this purpose 1 What is the 
only difficulty 1 How does the astronomical clock show us what por- 
tion of the equator passes under the meridian'? Do the parallels of 
latitude turn at the same rate with the equator 1 How do we meas- 
ure the difference of right ascension between two stars, by means 


and a quarter must have equalled forty-eight degrees 
and three quarters, which was the right ascension of 
the star. Two hours and three quarters afterward, 
that is, at six hours sidereal time, I observed another 
star cross the meridian. Its right ascension must have 
been ninety degrees, and consequently the difference 
of right ascension of the two, forty-one and a quarter 

212. Again, it is easy to take the declination of a 
body when on the meridian. By declination, we must 
recollect, is meant the distance of a body north or south 
of the celestial equator. When a star is crossing the 
meridian line of the transit instrument, the point of the 
meridian toward which the telescope is directed at 
that instant, will be shown on the graduated circle of 
the instrument, and the distance of that point from the 
zenith, subtracted from the latitude of the place of ob- 
servation, will give the decimation of the star. We 
have before seen, that when we have found the right 
ascensions and declinations of the heavenly bodies, we 
may lay down their relative situations on a map, just 
as we do those of places on the earth by their latitudes 
and longitudes. 

213. The Sextant is an instrument used for taking 
the angular distance of one point from another on the 
celestial sphere. It is particularly valuable for meas- 
uring celestial arcs at sea, because it is not, like most 
astronomical instruments, affected by the motion of the 
ship. The principle of the sextant may be briefly 
described as follows : it gives the angular distance be- 
tween any two objects on the celestial sphere, by re- 
flecting the image of one of the objects so as to coin- 

of the ctock'? Describe the mode of taking right ascensions with 
the transit instrument and clock. 

212. What is the declination of a body 1 How taken when on 
the meridian 1 

213. For what is the Sextant used 1 For what is it particularly 



cide with the other object as seen by the naked eye. 
The arc through which the reflector is turned to bring 
the reflected object to coincide with the other object, 
becomes a measure of the angular distance between 
them. The instrument is of a triangular shape, and 

Fig. 99. 

is made strong and firm by metallic cross-bars. It 
i has two small mirrors, I, H, called respectively, the 
I index glass and the horizon glass, both of which are 
firmly fixed perpendicularly to the plane of the in- 
strument. The index glass is attached to the movable 
arm, I D, and turns as this is moved along the gradu- 

i valuable 1 State its principle. Describe the Sextant. Point out 
| the Index glass and the Horizon glass. State the use of the Vernier. 


ated limb, E F. This arm carries a Vernier at D, a 
contrivance which enables us to read off minute parts 
of the spaces into which the limb is divided. The 
horizon glass, H, consists of two parts; the upper part 
being transparent or open, so that the eye looking 
through the telescope, T, can see through it a distant 
object, as a star, at S, while the lower part is a reflect- 
or. Suppose it were required to measure the dis- 
tance between the moon and a certain star, the moon 
being at M, and the star at S. The instrument is held 
firmly in the hand, so that the eye, looking -through 
the telescope, sees the star, S, through the transparent 
part of the horizon glass. Then the movable arm, 
I D, is moved from F toward E, until the image of M 
is reflected down to S ; when the number of degrees 
and parts of a degree reckoned on the limb from F to 
the index at D, will show the angular distance be- 
tween the two bodies. The altitude of the sun above 
the horizon, at any time, may be taken by looking di- 
rectly at the line of the horizon (which is well defined 
at sea) and moving the index from F toward E, until 
the limb of the sun just grazes the horizon. 



214. As time is a measured portion of indefinite du- 
ration, any thing or any event which takes place at 
equal intervals, may become a measure of time. But 
the great standard of time is the period of the revolu- 

Describe the Horizon glass. Describe the mode of taking an ol> 
eervation with the Sextant. How to take the sun's altitude. 

TIME. 203 

tion of the earth on its axis, which, by the most exact 
observations, is found to be always the same. The 
time of the earth's revolution on its axis, as already 
explained, is called a sidereal day, and is determined 
by the apparent revolution of a star in the heavens. 
This interval is divided into twenty-four sidereal hours. 
215. Solar time is reckoned by the apparent revolu- 
tion of the sun from noon to noon, that is, from the 
meridian round to the meridian again. Were the sun 
stationary in the heavens like a fixed star, the time of 
its apparent revolution would be equal to the revolu- 
tion of the earth on its axis, and the solar and sidereal 
days would be equal. But since the sun passes from 
west to east, in his apparent annual revolution around 
the earth, three hundred and sixty degrees in three 
hundred and sixty-five days, he moves eastward nearly 
a degree a day. While, therefore, the earth is turn- 
ing once on its axis, the sun is moving in the same di- 
rection, so that when we have come round under the 
same celestial meridian from which we started, we do 
not find the sun there, but he has moved eastward 
nearly a degree, and the earth must perform so much 
inore than one complete revolution, before our meri- 
dian cuts the sun again. Now, since we move in the 
diurnal revolution, fifteen degrees in sixty minutes, 
we must pass over one degree in four minutes. It 
takes, therefore, four minutes for us to catch up with 
the sun, after we have made one complete revolution. 
Hence, the solar day is almost four minutes longer than 
the sidereal ; and if we were to reckon the sidereal 
day twenty-four hours, we should reckon the solar day 
twenty-four hours and four minutes. To suit the pur- 

214. What may become a measure of time 1 What is the great 
standard of time 1 

215. Distinguish between sidereal and solar time. Why are the so- 
lar days longer than the sidereal 1 How much longer 1 If we count 
the solar day twenty-four hours, how long is the sidereal day 1 


poses of society at large, however, it is found more' 
convenient to reckon the solar day twenty-four hours, 
and throw the fraction into the sidereal day. Then, 

24h 4m : 24h : : 24h : 23h 56m 4s. 
That is, when we reduce twenty-four hours and four 
minutes to twenty-four hours, the same proportion will 
require that we reduce the sidereal day from twenty- 
four hours to twenty-three hours fifty-six minutes four 
seconds ; or, in other words, a sidereal day is such a ; 
part of a solar day. 

216. The solar days, however, do not always differ 
from the sidereal by precisely the same fraction, since 
they are not constantly of the same length. Time, as 
measured by the sun, is called apparent time, and a 
clock so regulated as always to keep exactly with the 
sun, is said to keep apparent time. But as the sun in 
his apparent motion round the earth once a year, goes 
sometimes faster and sometimes slower, a clock which 
always keeps with the sun must vary its motion ac- 
cordingly, making some days longer than others. The 
average length of all the solar days throughout the year, 
constitutes Mean Time. Clocks and watches are com- 
monly regulated to mean time, and therefore do not 
keep exactly with the sun, but are sometimes faster 
and sometimes slower than the sun. If one clock is j 
so constructed as to keep exactly with the sun, and" 
another clock is regulated to mean time, the difference 
between the two clocks at any period is the equation 
of time for that period. The two clocks would differ 
most about the third of November, when the apparent 
time is sixteen and a quarter minutes faster than the 
mean time. But since apparent time is at one time 
greater and at another less than mean time, the two 

216. Do the solar days always dilier from the sidereal by the same 
fraction 1 What is apparent time 1 When is a clock said to keep 
apparent time 1 What constitutes mean time 1 How are clocks and 
watches commonly regulated 1 What is the equation of time 1 
W T hen would the two ^clocks differ most, and how much ? When 
would they be together 1 


must obviously be sometimes equal to each other. 
This is the case four times a year ; namely, April 15th, 
June 15th, September 1st, and December 24th. 

217. As a day is the period of the revolution of the 
earth on its axis, so a year is the period of the revolu- 
tion of the earth around the sun. This time, which 
constitutes the astronomical year, has been ascertained 
with great exactness, and found to be 365d. 5h. 48m. 
51 sec. The ancients omitted the fraction, and reck- 
oned it only 365 days. Their year, therefore, would 
end about six hours before the sun had completed his 
apparent revolution in the ecliptic, and, of course, be 
so much too short. In four years they would disagree 
a whole day. This is the reason why every fourth 
year is made to consist of 366 days, by reckoning 29 
days in February instead of 28. This fourth year the 
ancients called Bissextile we call it Leap year. 
Fig. 100. 
O P 



218. PARALLAX is the apparent change of place which 
objects undergo by being viewed from different points. 

217. What period is a year 1 What is its exact length 7 How 
long did the ancients reckon it 1 Explain why every fourth year 
is reckoned 366 days. 




All objects beyond a certain moderate height above 
us, appear to be projected on the face of the sky ; but 
spectators at some distance from each other refer the 
same body to different points of the sky. Thus, if 
M N (Fig. 100) represents the sky, and C and D two 
bodies in the atmosphere, a spectator at A would refer 
C to M, while one at B would refer it to N. The 
arc, M N, would measure the angle of the parallax. 
In the same manner, O P would measure the angle 
of parallax of the body D. It is evident from the figure, 
that nearer objects have a much greater parallax than 
those that are remote. Indeed, the fixed stars are so 
distant, that two spectators a hundred millions of miles 
apart would refer a given star to precisely the same 
part of the heavens. But the moon is comparatively 
near, and her apparent place in the sky, at a given 
time, is much affected by 
parallax. Thus, to a spec- 
tator at A, the moon would 
appear in the sky at D, 
while to one at B, it would 
appear at C. Hence, since 
the same body often appears 
at the same time differently 
situated to spectators in dif- 
ferent parts of the earth, 
astronomers have agreed to 
consider the true situation 
of a body to be that where 
it would appear in the sky 
if viewed from the center 
of the earth. 

218. Define parallax. Where do all objects at a certain heieht 
appear to be projected '1 How is the same body projected by dif- 
ferent spectators 1 When have objects a large and when a small 
parallax 1 What is said of the fixed stars 1 Of the moon 1 What 
do astronomers consider the true place of a body 1 


219. The change of place which a body seen in the 
horizon, by a spectator on the surface of the earth, 
would undergo if viewed from the center, is called 
horizontal parallax. Although we cannot go to the 
center of the earth to view it, yet we can determine 
by the aid of geometry where it would appear if seen 
from the center, and hence we can find the amount of 
the horizontal parallax of a heavenly body, as the sun 
or moon. When we know the horizontal parallax of 
a heavenly body, we can ascertain its distance from us ; 
but the method of doing this cannot be clearly under- 
stood without some knowledge of trigonometry. 

220. REFRACTION is a change of place which the 
heavenly bodies seem to undergo, in consequence of the 
direction of their light being altered in passing through 
the atmosphere. As a ray of light traverses the atmo- 
sphere, it is constantly bent more and more, by the re- 
fraction of the atmosphere, out of its original direction. 
Now an object always appears in that direction in 
which the light from it finally comes to the eye. By 
refraction, therefore, the heavenly bodies are all made 
to appear higher than they really are, especially when 
they are near the horizon. The sun and moon, when 
near rising or setting, are elevated by refraction more 
than their whole diameter, so that they appear above 
the horizon both before they have actually risen and 
after they have set. 

221. TWILIGHT is that illumination of the sky which 
takes place before sunrise and after sunset, by means of 
which the day advances and retires by a gradual in- 
crease or diminution of the light. While the sun is 
within eighteen degrees of the horizon, some portion 

219. What is horizontal parallax 1 "What use is made of horizon- 
tal parallax 1 

220. Define refraction. How is a ray of light affected by travers- 
ing the atmosphere 1 How does refraction aflect the apparent places 
of the heavenly bodies 1 What is said of the sun and moon 1 . 


of its light is conveyed to us by means of the numerous 
reflexions from the atmosphere. At the equator, where 
the circles of daily motion are perpendicular to the 
horizon, the sun descends through eighteen degrees in 
an hour and twelve minutes. In tropical countries, 
therefore, the light of day rapidly declines, and as 
rapidly advances after daybreak in the morning. At 
the pole, a constant twilight is enjoyed while the sun 
is within eighteen degrees of the horizon, occupying 
nearly two-thirds of the half year, when the direct 
light of the sun is withdrawn, so that the progress 
from continual day to constant night is exceedingly 
gradual. To an inhabitant of one of the temperate 
zones, the twilight is longer in proportion as the place 
is nearer the elevated pole. 





222. THE distance of the sun from the earth is about 
ninety-five millions of miles. Although, by means 
of the sun's horizontal parallax, astronomers have been 
able to find this distance in a way that is entitled to 
the fullest confidence, yet such a distance as 95,000,000 
of miles seems almost incredible. Still it is but 
small compared with the distance of the fixed stars. 
Let us make an effort to form some idea of this vast 
distance, which we shall do best by gradual approaches 
to it. We will then begin with so small a distance 

221. Define twilight. How far is the sun below the horizon when 
the twilight ceases 1 How is it at the equator at the poles and in 
the middle latitudes 1 

222. Distance of the sun from the earth. How does it compare with 

THE SUN. 209 

as that across the Atlantic ocean, and follow in mind 
a ship, as she leaves the port of New York, and after 
twenty days' sail reaches Liverpool. Having formed 
the best idea we can of this distance, we may then 
reflect, that it would take a ship, moving constantly at 
the rate of ten miles an hour, more than a thousand 
years to reach the sun. 

223. The diameter of the sun is toward a million 
of miles ; or, more exactly, it is 885,000 miles. One 
hundred and twelve bodies as large as the earth, lying 
side by side, would be required to reach across the solar 
Usk ; and our ship, 'sailing at the same rate as before, 
would be ten years in passing over the same space. 
Immense as is the sun, we can readily understand why 
it appears no larger than it does, when we reflect that 
its distance is still more vast. Even large objects on 
the earth, when seen on a distant eminence, or over a 
wide expanse of waters, dwindle almost to a point. 
Could we approach nearer and nearer to the suri, it 
would constantly expand its volume until it finally filled 
the whole sky. We could, however, approach but little 
nearer the sun than we are, without being consumed 
by his heat. Whenever we come nearer to any fire, 
the heat rapidly increases, being four times as great 
at half the distance, and one hundred times as great 
at one tenth the distance. This fact is expressed by 
saying, that heat increases as the square of the distance 
decreases. Our globe is situated at such a distance 
from the sun, as exactly suits the animal and vegetable 
kingdoms. Were it either much nearer or more 
remote, they could not exist, constituted as they are. 
The intensity of the solar light also follows the same 

that of the fixed stars 1 Effort to form an idea of great distances. How 

long would it take a ship, moving ten miles an hour, to reach the sun 1 

223. Diameter of the sun. How many bodies like the earth would 

it take to reach across the sun 1 How long the ship to sail over it 1 

Why it appears no larger 1 How would it appear could we approach 

nearer and nearer to it * How is the intensity of heat proportioned 



law. Consequently, were we much nearer the sun 
than we are, its blaze would be insufferable ; or were 
we much farther off, the light would be too dim to serve 
all the purposes of vision. 

224. The sun is one million four hundred thousand 
(1,400,000) times as large as the earth ; but its matter 
is only about one fourth as dense as that of the earth, 
being only a little heavier than water, while the average 
density of the earth is more than five times that of 
water. Still, on account of the immense magnitude 
of the sun, its quantity of matter is 354,000 times as 
great as that of the earth. Bodies would weigh about 
twenty-eight times as much at the surface of the sun 
as they do on the earth. Hence, a man weighing three 
hundred pounds would, if conveyed to the surface of 
the sun, weigh 8,400 pounds, or nearly three tons and 
three quarters. A man's limb, weighing forty pounds, 
would require to lift it a force of 1,120 pounds, which 
would be beyond the ordinary power of the muscles. 
At the surface of the earth, a body falls from rest by 
the force of gravity, in one second, 16 T ' feet ; but at 
the surface of the sun, a body would, in the same time, 
fall through 449 feet. 

225. When we look at the sun through a telescope, 
we commonly find on his disk a greater or less number 
of dark places, called Solar Spots. Sometimes the 
sun's disk is quite free from spots, while at other times 
we may see a dozen or more distinct clusters, each 
containing a great number of spots, some large and 
some very minute. Occasionally a single spot is so 
large as to be visible to the naked eye, especially when 

to the distance 1 Were the earth nearer the sun, what would be the 
consequence 1 How would its light increase 7 

224. How much larger is the sun than the earth 1 How much 
greater is its quantity of matter 1 How much more would bodies 
weigh at the sun than at the earth 1 How much would a man of 
three hundred pounds weigh 1 Through what space would a body 
fall in a second 1 

225. Solar spots their number size of the largest their apparent 

THE SUN. 211 

.; the sun is near the horizon, and the glare of his light 

is taken off. Spots have been seen more than 50,000 

miles in diameter. They move slowly across the 

| central regions of the sun. As they have all a common 

motion from day to day across the sun's disk ; as they 

i go off on one limb, and, after a certain interval, some- 

I times come on again on the opposite limb, it is inferred 

that this apparent motion is imparted to them by an 

actual revolution of the sun on his axis, which is 

< accomplished in about twenty-five days. This is called 

: the sun's diurnal revolution, while his apparent move- 

I ment about the earth once a year is called his annual 

J revolution. 

226. We have seen that the apparent revolution of 

I the heavenly bodies, from east to west, every twenty- 

i four hours, is owing to a real revolution of the earth on 

I its own axis in the opposite direction. This motion is 

very easily understood, resembling, as it does, the 

! spinning of a top. We must, however, conceive of the 

Jtop as turning without any visible support, and not as 

resting in the usual manner on a plane. The annual 

j -motion of the earth around the sun, which gives rise to 

! the apparent motion of the sun around the earth once a 

year, is somewhat more difficult to understand. When, 

as the string is pulled, the top is thrown forward on 

| the floor, we may see it move onward (sometimes in a 

: circle) at the same time that it spins on its axis. Let 

a candle be placed on a table, to represent the sun, 

and let these two motions be imagined to be given 

to a top around it, and we shall have a case somewhat 

resembling the actual motions of the earth around the 



.; motions revolution of the sun. Distinction between the diurnal 
j and annual revolutions of the sun. 

226. To what is the apparent daily motion of the sun from east 
I to west owing 1 How to conceive of it 1 How to conceive of the 
I annual motion 1 


227. When bodies are at such a distance from each 
other as the earth and sun, a spectator on either would 
project the other body upon the face of the sky, always 
seeing it on the opposite side of a great circle, one 
hundred and eighty degrees from himself. Let Fig. 
Fig. 102. 

102 represent the relative positions of the earth and 
sun, and the firmament of stars. A spectator on the 
earth at |, (Aries,) would see the sun in the heavens - 
at =:, (Libra ;) and while the earth was moving from 
| to o5, (Cancer,) not being conscious of our own,-> 
motion, but observing the sun to shift his apparent place 
from . to VS, (Capricornus,) we should attribute the 
change to a real motion in the sun, and infer that the 
sun revolves about the earth once a year, and not the 
earth about the sun. Although astronomers have 
learned to correct this erroneous impression, yet they 
still, as a matter of convenience, speak of the sun's 
annual motion. 

227. How would a spectator on the sun or the earth, project the 
other body 1 Illustrate by the Figure. 

THE SUN. 213 

228. In endeavoring to obtain a clear idea of the 
revolution of the earth around the sun, imagine to 
yourself a plane (a geometrical plane, having merely 
length and breadth, but no thickness,) passing through 
the centers of the sun and earth, and extended far be-< 
yond the earth, until it reaches the firmament of stars. 
This is the plane of the ecliptic ; the circle in which 
it seems to cut the heavens is the celestial ecliptic ; and 
the path described by the earth in its revolution around 
,the sun, is the earth's orbit. This is to be conceived 
of as near to the sun compared with the celestial eclip- 
tic, although both are in the same plane. Moreover, 
we project the sun into the celestial ecliptic, because 
it seems to travel along the face of the starry heavens, 
I i since the sun and stars are both so distant that we ean- 
jnot distinguish between them in this respect, but see 
jthem both as if they were situated in the imaginary 
idome of the sky. If the sun left a visible trace on 
Ithe face of the sky, the celestial ecliptic would of 
course be distinctly marked on the celestial sphere, as 
jit is on an artificial globe ; and were the celestial equa- 

tor delineated in a similar manner, we should then see, 
at a glance, the relative position of these two circles ; 
the points where they intersect one another constituting 
ithe equinoxes; the points where they are at the greatest 
(distance asunder, being the solstices ; and the angle 
! which the two circles make with each other, (23 28',) 
being the obliquity of the ecliptic. 

229. As the earth traverses every part of her orbit 
sin the course of a year, she will be once at each sol- 
stice, and, once at each equinox. The best way of 

228. To obtain a clear idea of the revolution of the earth around 
ithe sun, what device shall we employ 1 What is the plane of the 
1 ecliptic 1 What the celestial ecliptic * What the earth's orbit 1 
Into what do we project the sun 1 If the sun left a visible track. 
: what would it mark out 1 If the celestial equator were delineated 
; in the same way, what would it mark out J Where would be the 
1 equinoxes the solstices 1 What is the obliquity 1 


obtaining a correct idea of her two motions, is to con- 
ceive of her as standing still a single day, at some 
point in her orbit, until she has turned once on her 
axis, then moving about a degree, and halting again 
until another diurnal revolution is completed. Let us 
suppose the earth at the Autumnal Equinox, the sun, of 
course, being at the Vernal Equinox. Suppose the 
earth to stand still in its orbit for twenty-four hours. 
The revolution of the earth on its axis, in this period, 
from west to east, will make the sun appear to de- 
scribe a great circle of the heavens from east to west, 
coinciding with the equator. At the end of this time, 
suppose the sun to move northward one degree in its 
orbit, and to remain there twenty-four hours, in which 
time the revolution of the earth will make the sun ap- 
pear to describe another circle from east to west, but 
a little north of the equator. Thus, we may conceive 
of the sun as moving one degree in the northern half 
of its orbit, every day, for about three months, when. 1 
he will reach the point of the ecliptic farthest from the 
equator, which point is called the tropic, from a Greek 
word signifying to turn; because, after the sun has 
passed this point, his motion in his orbit carries him 
continually toward the equator, and therefore he seems 
to turn about. The same point is also called the sol- 
stice, from a Latin word signifying to stand still; since, 
when the sun has reached its greatest northern or ' 
southern limit, he seems for a short time stationary, with 
regard to his annual motion, appearing for several days 
to describe, in his daily motion, the same parallel of 

230. When the sun is at the northern tropic, which 
happens about the 21st of June, his elevation above 
the southern horizon at noon is the greatest in the 

229. How to obtain a clear idea of the earth's two motions de- 
scribe the process why is the turning point called the tropic? 
Why the solstice 1 

THE SUN. 215 

year ; and when he is at the southern tropic, about 

the 21st of December, his elevation at noon is the least 

in the year. 

231. The motion of the earth, in its orbit, is nearly 
! seventy times as great as its greatest motion around 
'its axis. In its revolutions around the sun, the earth 
, moves no less than 1,640,000 miles a day, 68,000 
.miles an hour, 1,100 miles a minute, and 19 miles 
j every second a velocity sixty times as great as the 
i greatest velocity of a cannon ball. Places on the 

earth turn with very different degrees of velocity in 
; different latitudes. Those on the equator are carried 
| round at the rate of about 1000 miles an hour. In 
jour latitude, (41 18',) the diurnal velocity is about 
j 750 miles an hour. It would seem at first quite in- 
j credible that we should be whirled round at so rapid 

a rate, and yet be entirely insensible of any motion ; 

and much more that we should be going on so swiftly 
j through space, in our circuit around the sun, while 
J all things, when unaffected by local causes, appear to 
be in such a state of quiescence. Yet we have the 
| most unquestionable evidence of the fact ; nor is it 
j difficult to account for it, in consistency with the gene- 
1 I ral state of repose among bodies on the earth, when 
4 we reflect that their relative motions, with respect to 
j each other, are not in the least disturbed by any mo- 
| tions which they may have in common. When we 
j are on board a steamboat, we move about in the same 
i manner when the boat is in rapid motion, as when it 
| is lying still ; and such would be the case, if it moved 

steadily a hundred times faster than it does. Were 

230. When does the sun reach the northern tropic *? How is then 
i his altitude 1 When is he at the southern tropic 1 His altitude 
1 then * 

231. How much greater is the motion of the earth in its orbit than 
i on its axis 1 How many miles per day per hour per minute per 
' second 1 Rates of motion of places in different latitudes 1 Rate in 

latitude 41 degrees and 18 minutes 1 Why are we insenbible to this 


the earth, however, suddenly to stop its diurnal motion, 
all movable bodies on its surface would be thrown off 
in tangents to the surface, with velocities proportional 
to that of their diurnal motion ; and were the earth 
suddenly to halt in its orbit, we should be hurled for. 
ward into space with inconceivable rapidity. 

232. The phenomena of the SEASONS, which we 
may now explain, depend on two causes ; first, the' 
inclination of the earth's axis to the plane of its orbit ; 
and, secondly, to the circumstance that the earth's axis 
always remains parallel to itself. Imagine a candle, 
placed in the center of a large ring of wire, to repre- 
sent the sun in the center of the earth's orbit, and an 
apple with a knitting-needle running through it, in thai 
direction of the stem. Run a knife round the central, 
part of the apple, to mark the situation of the equator. 
The circumference of the ring represents the earth's 
orbit in the plane of the ecliptic. Place the apple so 
that the equator shall coincide with the wire ; then 
the axis will lie directly across the plane of the eclip- 
tic ; that is, at right angles to it. Let the apple be| 
carried quite round the ring, constantly preserving the] 
axis parallel to itself, and the equator all the while] 
coinciding with the wire that represents the orbitJ 
Now, since the sun enlightens half the globe at once, 
so the candle, which here represents the sun, will 
shine on the half of the apple that is turned toward! 
it ; and the circle which divides the enlightened fromj 
the unenlightened side of the apple, called the termi-\ 
nator, will pass through both the poles. If the applej 
be turned slowly round on its axis, the terminator will; 
pass successively over all places on the earth, giving! 

great motion 1 Illustrate by a steamboat. What would be the 
consequence were the earth suddenly to stop its motions 1 

232. What are the two causes of the change of seasons! Howl 
illustrated 1 How will the appearances be when the apple is so] 
placed that its equator coincides with the wire 1 Where will it be| 
sunrise where sunset 1 

THE SUN. 217 

the appearance of sunrise to places at which it arrives, 
and of sunset to places from which it departs. 

233. If, therefore, the earth's axis had been perpen- 
dicular to the plane of its orbit, in which case the equator 
would have coincided with the ecliptic, the diurnal 
motion of the sun would always have been in the equator, 
and the days and nights would have been equal all over 
the globe, and there would have been no change of 
seasons. To the inhabitants of the equatorial regions, 
the sun would always have appeared to move in the 
prime vertical, rising directly in the east, passing through 
the zenith at noon, and setting in the west. In the polar 
regions, the sun would always have appeared to revolve 
in the horizon ; while, at any place between the equator 
and the pole, the course of the sun would have been 
oblique to the horizon, but always oblique in the same 
degree. There would have been nothing of those 
agreeable vicissitudes of the seasons which we now 
enjoy ; but some regions of the earth would have been 
crowned with perpetual spring ; others would have 
been scorched with a burning sun continually over- 
head ; while extensive regions toward either pole, 
would have been consigned to everlasting frost and 

234. In order to simplify the subject, we have just 
supposed the earth's axis to be perpendicular to the 
plane of its orbit, making the equator to coincide with 
the ecliptic ; but now, (using the same apparatus as 
before,) turn the apple out of a perpendicular position 
a little, (231 degrees,) then the equator will be turned 
just the same number of degrees out of a coincidence 
with the ecliptic. Let the apple be carried around the 

233. Comparative lengths of the days and nights * Appearances 
to the inhabitants of the equatorial regions 1 Of the polar regional 
Would there have been any change of seasons 1 

234. Repeat the process with the axis inclined. How far would 
the equator be turned out of a coincidence with the ecliptic 1 How 




ring, always holding it inclined at the same angle to 
the plane of the ring, and always parallel to itself, as 

in figure 103. We shall find that there are two points, 
A and C, in the circuit, where the light of the sun 
(which always enlightens half the globe at once) 
reaches both poles. These are the points where the 
celestial equator and ecliptic cut one another, or the 
equinoxes. When the earth is at either of these points, 
the sun shines on both poles alike ; and if we conceive 
of the earth, while in this situation, as turning once 
round on its axis, the apparent diurnal motion of the 
sun would be the same as it would be, were the earth's 
axis perpendicular to the plane of the equator. For 
that day, the earth would appear to revolve in the .] 
equator, and the days and nights would be equal all 
over the globe. 

235. If the apple were carried round in the manner 
supposed, then, at the distance of ninety degrees from ] 
the equinoxes, at B and D, the same pole would bel 
turned toward the sun on one side, just as much as it 

does the sun then shine with respect to the poles 1 What will then 
fee the appearances in the diurnal motion 1 

THE SUN. 219 

was turned from him on the other. In the former case, 
the sun's light would reach beyond the pole 23^ degrees, 
and in the other case, it would fall short of it the same 
number of degrees. Now imagine, again, the earth 
turning in the daily revolution, and it will be readily 
seen how places within 23^ degrees of the enlightened 
pole, will have continual day, while places within the 
same distance of the unenlightened pole, will have 
j continual night. By an attentive inspection of figure 
1 103, all these things will be clearly understood. The 
earth's axis is represented as prolonged, both to show 
I its position, and to indicate that it always remains 
parallel to itself. On March 21st and September 22d, 
when the earth is at the equinoxes, the sun shines on 
both poles alike ; while on June 21st and December 
24th, when the earth is at the solstices, the sun shines 
23^ degrees beyond one pole, and falls the same distance 
short of the other. 

236. Two causes contribute to increase the heat of 
summer and the cold of winter, the changes in the 
sun's meridian altitudes > and in the lengths of the days. 
The higher the sun ascends above the horizon, the more 
directly his rays fall upon the earth ; and their heating 
power is rapidly increased as they approach a perpen- 
dicular direction. The increased length of the day in 
jsummer, affects greatly the temperature of places 
toward the poles, because the inequality between the 
lengths of the day and night is greater in proportion 
as we recede from the equator. By the operation of 
{this cause, the heat accumulates so much in summer, 
that the temperature rises to a higher degree in mid- 
summer, at places far removed from the equator, than 
within the torrid zone. 

235 At the distance of 90 degrees from the equinoxes, how would 
the sun shine with reppect to tfie poles'? 

236 What two causes contribute to increase the heat of summer 
and the ^old of winter 1 Effect of the sun's altitude of the increased 
length of the day 1 



237. But the temperature of a place is influence' 
very much by several other causes, as well as by th 
force and duration of the sun's heat. First, the eleva- 
tion of a country above the level of the sea, has a 
great influence upon its climate. Elevated districts of ; 
country, even in the torrid zone, often enjoy the most! 
agreeable climate in the world. The cold of the upper j 
regions of the atmosphere modifies and tempers the j 
solar heat, so as to give a most delightful softness, while i. 
the uniformity of temperature excludes those sudden 
and excessive changes which are often experienced in 
less favored climes. In ascending high mountains, 
situated within the torrid zone, the traveller passes, hv 
a short time, through every variety of climate, from < 
the most oppressive and sultry heat, to the soft and '' 
balmy air 01 spring, which again is succeeded by the 
cooler breezes of autumn, and then by the severest 
frosts of winter. A corresponding difference is seen in > 
the products of the vegetable kingdom. While winter 
reigns on the summit of the mountain, its central regions ; 
may be encircled with the verdure of spring, and itsj^ 
base with the flowers and fruits of summer. Secondly, 
the vicinity of the ocean has also a great effect to; 1 
equalize the temperature of a place. As the ocean 
changes its temperature during the year much less than a 
the land, it becomes a source of warmth to neighboring! 
countries in winter, and a fountain of cool breezes inA 
summer. Thirdly, the relative moisture or dryness off 1 ' 
the atmosphere of a place is of great importance, in' 
regard to its effects on the human system. A dry air|| 
of ninety degrees, is not so insupportable as a moisli 
air of eighty degrees. As a general principle, a hot ' 
and moist air is unhealthy, although a hot air, when 
dry, may be very salubrious. 

237. Effect of elevation of the vicinity of the ocean relative 
moisture and dryness. 




238. THE Moon is a constant attendant or satellite 
of the earth, revolving around it at the distance of 
about 240,000 miles. Her diameter exceeds 2,000 
miles, (2160.) Her angular breadth is about half a 
degree, a measure which ought to be remembered, 
as it is common to estimate fire-balls, and other sights 
in the sky, by comparing them with the size of the 
moon. The sun's angular diameter is a little greater. 

239. When we view the moon through a good tel- 
escope, the inequalities of her surface appear much 
more conspicuous than to the naked eye ; and by stu- 
dying them attentively, we see undoubted proofs that 
the face of the moon is very rough and broken, exhib- 

\ iting high mountains and deep valleys, and long moun- 
tainous ridges. The line which separates the enlight- 
ened from the dark part of the moon, is called the 
\ Terminator. This line appears exceedingly jagged, 
\ indicating that it passes over a very broken surface 
of mountains and valleys. Mountains are also indi- 
cated by the bright pointe and crooked lines, which 
lie beyond the terminator, within the unilluminated 
part of the moon ; for these can be nothing else than 
elevations above the general level, which are enlight- 
ened by the sun sooner than the surrounding countries, 
as high mountains on the earth are tipped with the 
morning light sooner than the countries at their bases. 
Moreover, when these pass the terminator, and come 

238. Of what is the moon a satellite 1 Distance from the earth 
diameter angular breadth. Why is it important to remember this *? 

239 How does the moon appear to the telescope 1 What is the 
Terminator 1 How does it appear 1 What does its unevenness in- 
dicate 1 What signs of mountains are there in the dark part of the 



within the enlightened part of the disk, they are fur- 
ther recognised as mountains, because they cast shad- 
ows opposite the sun, which vary in length as the sun 
strikes them more or less on a level. 

Fig. 104. 

240. Spots, also, on the lunar disk, are known to be 
Valleys, because they exhibit the same appearance as 
is seen when the sun shines into a tea cup, when it 
strikes it very obliquely. The inside of the cup, oppo* 
site to the sun, is illuminated in the form of a crescent, 

moon'? When the terminator passes beyond these^ what signs 01 
being mountains do they give 1 
240. Valleys, how known. Illustrate by the mode in which lighl 

THE MOON. 223 

(as every one may see, who will take the trouble to 
try the experiment,) while the inside, next the sun, 
casts a deep shadow. Also, if the cup stands on a 
table, the side farthest from the sun casts a shadow 
on the table outside of the cup. Similar appearances, 
presented by certain spots in the moon, indicate very 
clearly that they are valleys. Many of them are reg- 
ular circles, and not unfrequently we may see a chain 
of mountains, surrounding a level plain of great ex- 
tent, from the center of which rises a sharp mountain, 
casting its shadow on the plain within the circle. 
Figure 104 is an accurate representation of the tele- 
scopic appearance of the moon when five days old. 
! It will be seen that the terminator is very uneven, and 
that white points and lines within the unenlightened 
part of the disk, indicate the tops of mountains and 
j mountain ridges. Near the bottom of the terminator, 
i a little to the left, we see a small circular spot, sur- 
I rounded by a high chain of mountains, (as is indicated 
j by the shadows they cast,) and in the center of the 
I valley the long shadow of a single mountain thrown 
upon the plain. Just above this valley, we see a ridge 
of mountains, casting uneven shadows opposite to the 
sun, some sharp, like the shadows of mountain peaks. 
These appearances are, indeed, rather minute ; but we 
j must recollect that they are represented on a very 
j small scale. The most favorable time for viewing the 
I mountains and valleys of the moon with a telescope, 
I is when she is about seven days old. 

241. The full moon does not exhibit the broken as- 
* pect so well as the new moon ; but we see dark and 
| light regions intermingled. The dusky places in the 
| moon were formerly supposed to consist of water, and 

| shines into a cup. What shape have many of the valleys 1 What 
<>} do we sometimes see surrounding the valley 1 What rising in the 
center of it 1 Point out mountains and valleys on the diagram. 
241. What is said of the telescopic view of the full moon 1 What 


the brighter places, of land ; astronomers, however, 
are now of the opinion, that there is no water in the 
.moon, but that the dusky parts are extensive plains, 
while the brightest streaks are mountain ridges. Each 
separate place has a distinct name. Thus, a remark- 
able spot near the top of the moon, is called Tycho ; 
another, Kepler ; and another, Copernicus ; after cel- 
ebrated astronomers of these names. The large 
dusky parts are called seas, as the Sea of Humors, 
the Sea of Clouds, and the Sea of Storms. Some of 
the mountains are estimated as high as five miles, and 
some of the valleys four miles deep. 

242. The moon revolves about the earth from west 
to east, once a month, and accompanies the earth ; 
around the sun once a year. The interval in which 
she goes through the entire circuit of the heavens, 
from any star round to the same star again, is called a 
sidereal month, and consists of about 27 days ; but the 
time which intervenes between one new moon and 
another, is called a synodical month, and is composed 
of 29^ days. A new moon occurs when the sun and, 
moon meet in the same part of the heavens ; for al- 
though the sun is 400 times as distant from us as the 
moon, yet as we project them both upon the face of the 
sky, the moon seems to be pursuing her path among 
the stars as well as the sun. Now the sun, as well as 
the moon, is travelling eastward, but with a slower 
pace ; the sun moves only about a degree a day, while ' 
the moon moves more than thirteen degrees a day. ' 
While the moon, after being with the sun, has been ' 
going round the earth in 27 days, the sun, mean-': 

were the dark places in the moon formerly supposed to be *? What 
do astronomers now consider them 1 How are places on the moon 
named 1 Repeat some of the names. What is the height of some 
of the mountains, and depth of the valleys'? 

242 Revolutions of the moon. What is a sidereal month 1 How 
long is it 1 What is a synodical month 1 When does a new moon 
occurl Why is the synodical longer than the sidereal month 1 

THE MOON. 225 

\vhile, has been going eastward about 27 degrees ; so 
that, when the moon returns to the part of the heavens 
where she left the sun, she does not find him there, but 
takes more than two days to catch up with him. 

243. The moon, however, does not pursue precisely 
I the same track with the sun in his apparent annual 

motion, though she deviates but little from his path. 
The inclination of her orbit to the ecliptic is only 
about five degrees, and, of course, the moon is never 
seen farther from the ecliptic than that distance, 
,and she is commonly much nearer to it than that. 
:The two points where the moon's orbit crosses the 
! ecliptic, are called her nodes. They are the intersec- 
| tions of the solar and lunar orbits, as the equinoxes are 
ithe intersections of the equator and ecliptic, and, like 
I the latter, are 180 degrees apart. 

244. The changes of the moon, Fig. 105. 
j commonly called her phases, arise 

; from different portions of her en- 

j lightened side being turned toward 

jthe earth at different times. When 

j the moon comes between the earth 

1 and the sun, her dark side is turned 

\ toward us, and we lose sight of her ^ 

I for a short period, at A, (Fig. 105,) 

j when she is said to be in conjunc- 
tion. As soon as she gets a little C c 
past conjunction, at B, we first 
observe her in the evening sky, Q ^ 

' in the form of a crescent, the Jk 

j well known appearance of the new 

j moon. When at C, half her enlightened disk is turned to- 

I ward us, and she is in quadrature, or in her first quarter. 

243. How many degrees is the moon's orbit inclined to the eclip- 
tic 1 Define the nodes. How far apart 1 

!244. Whence arise the phases of the moon 7 When is the moon 
said to be in conjunction 1 When in quadrature 1 When in oppo- 


At D, three-fourths of the disk is illuminated, and at E, 
when the earth lies between the sun and the moon, her 
whole disk is enlightened, and she is in opposition, thi 
time of full moon. In proceeding from opposition t 
conjunction, or from full to new moon, the illuminated 
portion diminishes in the same way as it increased front 
conjunction to opposition, being in the last quarter, at 
G. Within the first and last quarters, the terminator 
is turned from the sun, and the moon is said to be* 
horned; but within the second and third quarters, the' 
terminator presents its concave side toward the sun,' 
and the moon is said to be gibbous. 

245. The moon turns on her axis in the same time* 
in which she revolves about the earth. This is known: 
by the moon's always keeping nearly the same facei 
toward us, as is indicated by the telescope, which 
could not be the case, unless her revolution on her 
axis kept pace with her motion in her orbit. Take an 
apple to represent the moon : thrust a knitting-needle 
through it in the direction of the stern, to represent the 
axis, in which case the two eyes of the apple will nat- 
urally represent the poles. Through the poles, cut a 
line around the apple, dividing it into two hemispheres, 
and mark them so as to be readily distinguished from 
each other. Now place a ball on the table to repre- 
sent the earth, and holding the apple by the knitting, 
needle, carry it round the ball, and it will be seen that, 
unless the apple is made to turn about on its axis, as 
it is carried around the ball, it will present different 
sides toward the ball ; and that, in order to make it? 
always present the same side, it will be necessary to 
make it revolve exactly once on .its axis, while it is 

sitionl What figure has the moon in the first and last quarters! 
What in the second and third 1 

245. In what time does the moon turn on her axis 1 How is 
this known 1 How illustrated by an apple with a knitting-needle 1 
By walking round a tree 1 

THE MOON. 227 

gointf round the circle, the revolution on its axis 
keeping exact pace with the motion in its orbit. The 
same thing will be observed, if we walk around a tree, 
always keeping the face toward the tree. It will be 
; necessary to turn round on the heel at the same rate 
as we go forward round the tree. 

246. An Eclipse of the Moon happens when the 
moon, in its revolution around the earth, falls into the 
earth's shadow. An Eclipse of the Sun happens when 
the moon, coming between the earth and the sun, cov- 
ers either a part or the whole of the solar disk. As 
the direction of the earth's shadow is, of course, op- 
posite to the sun, the moon can fall into it only when 
in opposition, or at the time of full moon ; and as the 
moon can come between us and the sun only when in 
conjunction, or at the time of new moon, it is only 
then that a solar eclipse can take place. If the moon's 
orbit lay in the plane of the ecliptic, we should have 
a solar eclipse at every new moon, and a lunar eclipse 
at every full moon ; but as the moon's orbit is inclined 
to the plane of the ecliptic about five degrees, the 
moon may pass by the sun on one side, and the earth's 
shadow on the other side, without touching either. It 
is only when, at new moon, the sun happens to be at 
j or near the point where the lunar orbit cuts the plane 
j of the ecliptic, or at one of the nodes, that the moon's 
disk overlaps the sun's, and produces a solar eclipse, 
j Also, when the sun is at or near one of the moon's 
j nodes, the earth's shadow is thrown across the other 
[node, on the opposite side of the heavens, and then, 
las the moon passes through this node, at the time of 
(opposition, she falls within the shadow, and produces 
ja lunar eclipse. 

| 246. When does an eclipse of the moon happen *? When an 
I eclipse of the sun! At what age of the moon does it eclipse the 
) sun and at what age does it suffer eclipse 1 Why do not eclipses 
i occur at every revolution 1 At or near what point must the sun be, 
I in order that an eclipse may take place 1 


247. Figure 106 represents both kinds of eclipses. 
The shadow of the moon, when in conjunction, is 
represented as just long enough to reach the earth, as 

Fig. 106. 

is the case when the moon is at or about her average 
distance from the earth. In this case, a spectator on 
the earth, situated at the place where the point of the 
shadow touches the earth, would see the sun totally 
eclipsed for an instant, while the countries around, for 
a considerable distance, would see only a partial 1 
eclipse, the moon hiding only a part of the sun, which 
sheds on such places a partial light, called the penum- * 
bra, as is indicated in the figure by the dark shading 
on each side of the moon's shadow. A similar pe- 
numbra is represented on each side of the earth's 
shadow, because, when the moon is approaching the 
shadow, a part of the light of the sun begins to be in- 
tercepted from her when she reaches this limit, and 

247. Describe Fig. 106. At what point of the earth would the 
eclipse of the sun be total 1 Where partial 1 What is this partial 
light called 1 What, is said of the moon's penumbra 1 What oc- 
casions an annular eclipse 1 

THE MOON. 229 

she receives less and less of light from the sun, until, 
when she enters the shadow, his disk is entirely hidden. 
When the moon is farther from the earth than her 
average distance, her disk is not large enough to cover 
the sun's, but a ring of the sun appears all around the 
moon, constituting an annular eclipse. 

248. Eclipses of the sun are more frequent than 
those of the moon. Yet, lunar eclipses, being visible 
to every part, of the hemisphere of the earth in which 
the moon is above the horizon, while those of the sun 
are visible only to a small portion of the hemisphere 
on which the moon's shadow falls, it happens that, for 
any particular place on the earth, there are seen more 
eclipses of the moon than of the sun. In any year, the 
number of eclipses of both luminaries cannot be less 
than two, nor more than seven. The most usual 
number is four, and it is very rare to have more than 
six. A total eclipse of the moon frequently happens 
at the next full moon after an eclipse of the sun. For, 
since, in a solar eclipse, the sun is at or near one of 
the moon's nodes, that is, is projected to the place in 
the sky where the moon crosses the ecliptic, the earth's 
shadow, which is, of course, directly opposite to the 
sun, must be at or near the other node, and may not 
have passed too far from the node before the moon 
comes round to the opposition and overtakes it. 

249. A total eclipse of the sun is one of the most 
sublime and impressive phenomena of Nature. Among 
barbarous tribes, it is always looked on with fear and 
astonishment, and as strongly indicative of the wrath 
of the gods. When Columbus first discovered America, 

248. Which are most frequent, the eclipses of the sun or the moon 1 
Of which are the greatest number visible 1 What number of both 
can happen in a single year 1 What is the most usual number 1 
Why does an eclipse of the moon happen at the next full moon 
after an eclipse of the sun 1 

249. What is said of an eclipse of the sun 1 What ia t9ld of 
Columbus 1 Why is a total eclipse of the sun regarded with so 



and was in danger of hostility from the natives, he v 
awed them into submission by telling them that the sun 
would be darkened on a certain day, in token of the;| 
anger of the gods at them for their treatment of him. : 
Among cultivated nations, also, a total eclipse of the f 
sun is regarded with great interest, as verifying with 
astonishing exactness the predictions of astronomers, 
and evincing the great knowledge they have acquired of 
the motions of the heavenly bodies, and of the laws by 
which they are governed. From 1831 to 1838, was aj 
period distinguished for great eclipses of the sun, ia| 
which time there were no less than five, of the mos 
remarkable character. The next total eclipse of the 
sun, visible in the United States, will occur on the 7th of 
August, 1869. 

250. Since Tides are occasioned by the influence 
of the sun and moon, a few remarks upon them will I 
conclude the present chapter. By the tides are meant 
the alternate rising and falling of the waters of the 
ocean. Its greatest and least elevations are called high 
and low water ; its rising and falling are called flood and 
ebb ; and the extraordinary high and low tides that 
occur twice every month, are called spring and neap- f 
tides. It is high water, or low water, on opposite sides^ 
of the globe at the same time. If, for example, we have-1 
high water at noon, it is also high water to those who 
live on the meridian below us, where it is midnight.^ 
In like manner, low water occurs at the same time on| 
the upper and lower meridian. The average height of 1 
the tides, for the whole globe, is about two and a ban 
feet ; but their actual height at different places is very", 
various, sometimes being scarcely perceptible, and 

much interest among cultivated nations 1 What period was distin-^ 
guished for great eclipses of the sun 1 When will the next total 
eclipse of the sun occur 1 

250. What are the tides 1 What is meant by high and low water 
flood and ebb spring and neap 1 Where is it high water and 

THE MOON. 231 

ilsometimes rising to sixty or seventy feet. In the Bay 
rof Fundy, where the tide rises 70 feet, it comes in a 
imighty wave, seen thirty miles off, and roaring with a 
;loud noise. 

251. Tides are caused by the unequal attraction of 
ithe sun and moon upon different parts of the earth. We 
jshall attend hereafter more particularly to the subject 
jof universal gravitation, by which all bodies, or masses 
jof matter, attract all other bodies, each according to its 
jweight, when they act on a body at the same distance ; 
but when at different distances, the force increases 
irapidly as the distance is diminished, so that the force 
of attraction is four times as great for half the distance, 
jone hundred times as great for one tenth the distance, 
land, universally, the force increases in proportion as 
ithe square of the distance diminishes. Such a force as 
I this is exerted by the moon and by the sun upon the 
earth, and causes the tides. As the sun has vastly 
more matter than the moon, it would raise a higher tide 
I than the moon, were it not so much farther off. This 
latter circumstance gives the advantage to the moon, 
which has three times as much influence as the sun in 
raising the tides. If these bodies, one or both of them, 
acted equally on all parts of the earth, they would draw 
i all parts toward them alike, but would not at all disturb 
the mutual relation of the parts to each other, and, of 
course, would raise no tide. But the sun or moon 
attracts the water on the side nearest to it more than 
the water more remote, and thus raises them above the 
general level, forming the tide wave, which accompanies 
I the moon in her daily revolution around the earth. It 
is not difficult to see how the tide is thus raised on the 

where low water at the same time 1 Average height of the tides 
for the whole globe. What is said of their actual height at different 
places 1 How high does the tide rise in the Bay of Fundy * 

251. By what are tides caused 1 What force is exerted by the sun 
and moon upon the earth 1 Why does not the sun raise a greater 
tide than the moon 1 How does the sun's greater distance give the 


side of the meridian nearest to the moon ; but it may 
not be so clear why a tide should at the same time be 
raised on the opposite meridian. The reason of this is, 
that the waters farthest from the moon, being attracted 
less than those that are nearer, and less than the solid 
earth, are left behind, or appear to rise in a direction 
opposite to the center of the earth. Hence, we have | 
two tides every twenty-four hours, one when the ] 
moon passes the upper meridian, and one when she 
passes the lower. Each, however, is about fifty 
minutes later to-day than yesterday, for the moon i 
comes to the meridian so much later on each following 

252. Were it not for the impediments which prevent 
the force from producing its full effects, we might expect 
to see the great tide wave always directly beneath the 
moon, attending it regularly around the globe. But the 
inertia of the waters prevents their instantly obeying 
the moon's attraction, and the friction of the waters on 
the bottom of the ocean still further regards its progress. 
It is not, therefore, until several hours after the moon 
has passed the meridian of a place, that it is high tide 
at that place. 

253. The sun has an action similar to that of the 
moon, but only one third as great. It is not that the 
moon actually exerts a greater force of attraction upon j 
the earth than the sun does, that her influence in raising * 
the tides exceeds that of the sun. She, in fact, exerts ( \ 
much less force. But, being so near, the difference off 
her attraction on different parts of the earth is greater 
than the difference of the sun's attraction ; for the 
sun is so far off, that the diameter of the earth 

advantage to the moon'? Why is it high tide on opposite sides of-^ 
the earth at the same time 1 How much later is the high tide of I 
to-day than that of yesterday 1 

252. Why is it not high tide when the moon is on the meridian 1 

253. How much less is the action of the sun in raising the tides 
than that of the moon 1 Why has the moon so much greater power 1 

THE MOON. 233 

bears but a small proportion to the distance, and there- 
fore the force exerted by tfre sun is more nearly equal 
on all parts of the earth, and we must bear in mind 
that the tides are owing, not to the amount of the force 
of attraction, but to the difference of the forces exerted 
on different parts of the earth. 

254. As the sun and moon both contribute to raise 
the tides, and as they sometimes act together and 
sometimes in opposition to each other, so correspond- 
ing variations occur in the height of the tides. The 
spring tides, or those which rise to an unusual height 

Fig. 107. 

twice a month, are produced by the sun and moon's 
acting together ; and the neap tides, or those which 
are unusually low twice a month, are produced by the 
sun and moon's acting in opposition to each other. 
The spring tides occur when the sun and moon act in 
the same line, as is the case both at new and full 

Fig. 108. 

moon ; and the neap tides when the two luminaries 
act in directions at right angles to each other, as is the 

254. Explain the spring tides also the neap tides. Illustrate by 
the figures. 



case when the moon is in quadrature. The mode of 
action, in each case, will be clearly understood by in- 
specting Figs. 107 and 108. 

Fig. 107 shows the situation of the two luminaries 
when they act together at new moon. The waters are 
elevated both on the same side of the earth as the at- 
tracting bodies at A, and also on the opposite side, at 
B. If we now conceive the moon to change its place 
to B, when it would be full moon, the waters would 
still have the same elongated figure in the line of the 
two bodies, while at places 90 distant, at C and D, it^ 
would be low water. Again, in Fig. 108, the moon 
being in quadrature at C, the two attracting bodies act ; 
in opposition to each other, the sun raising a tide at A 
and B, while the moon raises a still higher tide at C ] 
and D. Hence, the high tide beneath the moon, and 
the low tide at places 90 distant, are both less than 

255. The largest lakes and inland seas have no per- 
ceptible tides. This is asserted by all writers respect- 
ing the Caspian and Black seas ; and the same is found 
to be true of the largest of the North American lakes, 
Lake Superior. Although these several tracts of wa- 
ter appear large, when taken by themselves, yet they 
occupy but small portions of the surface of the globe, 
as will be evident on seeing how small a space they 
occupy on the artificial globe ; so that the attraction 
of the sun and moon is nearly equal on all parts of 
such sea or lake. But it is the inequality of attraction 
on different parts that produces the tides. 

255. "Why have lakes and inland seas no tides 1 



SECTION 1. General View of the Planets. 

256. THE name planet is derived from a Greek word 
which signifies a wanderer, and is applied to this class 
of bodies, because they shift their positions in the heav- 
ens, whereas the fixed stars constantly maintain the 
same places with respect to each other. The planets 
known from a high antiquity are, Mercury, Venus, 
Earth, Mars, Jupiter, and Saturn. To these, in 1781, 
was added Uranus, (or Herschel, as it is sometimes 
called, from the name of the discoverer.) and, as late 
as the commencement of the present century, four more 
were added, namely, Ceres, Pallas, Juno, and Vesta. 
All these are called primary planets. Several of them 
have one or more attendants, or satellites, which revolve 
around them, as they revolve around the sun. The 
Earth has one satellite, namely, the Moon ; Jupiter has 
four, Saturn seven, and Uranus six. Mercury, Venus, 
and Mars, are without satellites. The same is the case 
with the four new planets, or asteroids, as they are 
sometimes called. The whole number of planets, 
therefore, is twenty-nine, namely, eleven primary, and 
eighteen secondary planets. 

257. Mercury and Venus are called inferior planets, 
because they have their orbits nearer to the sun than 
that of the earth ; while all the others, being more dis- 
tant from the sun than the earth is, are called superior 

256. Whence the name planet 1 What planets have been known, 
from a high antiquity 1 What have been added to these 1 What 
is said of the satellites 1 What is the whole number of planets 1 

257. Why are Mercury and Venus called inferior planets 1 Why 
the others superior planets ^ 



planets. Let us now compare the planets with 
another, in regard to their distances from the sun, the 
magnitudes, and their times of revolution. 

258. Distances from the sun, in miles. 

1. Mercury, 


2. Venus, 



3. Earth, 


4. Mars, 



5. Vesta, 



6. Juno, 


7. Ceres, 


8. Pallas, 


9. Jupiter, 



10. Saturn, 



11. Uranus, 



The dimensions of the planetary system are seen 
from this table to be vast, comprehending a circular 
space thirty-six hundred millions of miles in diameter. 
A railway car, travelling constantly at the rate of 
twenty miles an hour, would require more than twenty 
thousand years to cross the orbit of Uranus. 
259. Magnitudes. 



























We perceive that there is a great diversity among 
the planets, in regard to size. While Venus, an infe- 
frior planet, is nine-tenths as large as the Earth, Mars, 
-a superior planet, is only one-seventh, while Jupiter 
is twelve hundred and eighty-one times as large.* 

* The magnitudes are proportioned to the cubes of the diameters. 

258. Repeat the table of distances. "What is said of the dimen- 
:sions of the planetary system 1 How long would a railway car be 
in crossing the orbit of Uranus 1 

259. Repeat the table of magnitudes. What is said of the diversity 


Although several of the planets, when nearest to us, 
appear brilliant and large when compared with most 
of the fixed stars, yet the angle under which they are 
seen is very small, that of Venus, the greatest of all, 
never exceeding about one minute, which is less than 
one thirtieth the apparent diameter of the sun or moon.* 
Jupiter, also, by his superior brightness, sometimes 
makes a striking figure among the stars ; yet his greatest 
apparent diameter is less than one fortieth that of the 

260. Periodic Times. 

Mercury, 3 months. 

Venus, 7| " 

Earth, 1 year. 

Mars, 2 years. 

Ceres, 4| years. 
Jupiter, 12 " 
Saturn, 29 

Uranus, 84 

We perceive that the planets nearest the sun move 
most rapidly. Mercury performs nearly three hundred 
and fifty revolutions while Uranus performs one. The 
apparent progress of the most distant planets around 
the sun is exceedingly slow. Uranus advances only a 
little more than four degrees in a whole year ; so that 
we find this planet occupying the same sign, and of 
course remaining nearly in the same part of the heavens, 
for several years in succession. 

SEC. 2. Of the Inferior Planets. 

261. Mercury and Venus have their orbits so far 
within that of the earth, that they appear to us as 
attendants upon the sun. Both planets appear either 
in the west a* little after sunset, or in the east a little 

* In every estimation of angular breadths or distances, it is convenient to 
bear in mind that the angular breadth of the sun or moon is about half a 

in regard to size 1 What of the angular diameter of the planets 1 
How do the largest compare with the sun or moon 1 

260. Repeat the table of periodic times. What is said of the 
planets nearest the sun 1 What of those most distant 1 

261. How do Mercury and Venus appear with respect to the sun 1 



before sunrise. In high latitudes, where the twilight i 
long, Mercury can seldom be seen with the naked eye, 
and then only when its angular distance from the su 
is greatest. In our latitude, we can usually catch a 
glimpse of this planet for several evenings and morn- 
ings, if we will watch the time (usually given in the 
almanac) when it is at its greatest elongations from the 
sun. It, however, soon runs back again to the sun. 
The reason of this will be plain from the following 
diagram. Let S represent the sun, E the earth, 


M Q N R the orbit of Mercury, O Z P an arc of the 
heavens. Then, since we refer all distant bodies in the 
sky to the same concave sphere, we should see the sun 
at Z, in the heavens, and when the planet was at R or 
Q, we should see it close by the sun, and when it was 

What of Mercury in high latitudes 1 What in our latitude 1 When 
do we catch a glimpse of it 1 Explain the reason of this from the 


at its greatest elongation, at M or N, we should see it 
at O or P, when its angular distance from the sun 
would be measured by the arc O Z or P Z. Suppose 
Mercury comes into view at M, its greatest eastern 
elongation ; as it passes on to Q, its inferior conjunction, 
it appears to move in the sky backward, or contrary to 
the order of the signs, from O to Z ; and it continues 
its backward motion from M to N, or apparently from 
O to P. But now from N, its greatest western elongation, 
through R, its superior conjunction, to M, its greatest 
eastern elongation, its apparent motion is direct. Then, 
the planet is said to be in its superior conjunction. The 
inferior planets, Mercury and Venus, appear to run 
backward and forward across the sun, Mercury 
receding so little from that luminary as almost always 
to be lost in his beams. Venus, however, moves in a 
larger orbit, and recedes so far from the sun, on both 
sides, as often to remain a long time in the evening 
or morning sky, always immediately following or pre- 
ceding the sun, and hence called the evening and 
morning star. 

262. When an inferior planet is near its greatest 
elongation, on either side, it presents to us, when viewed 
with the telescope, half its enlightened disk, appearing 
to the telescope like the moon in one of her quarters. 
While passing from the eastern to the western elonga- 
tion, through the inferior conjunction, the enlightened 
portion grows less and less, taking the crescent form, 
like the old of the moon, until it arrives at the inferior 
conjunction, when it presents the entire dark side 
toward us. Soon after passing the conjunction, it 
appears like the new moon, and increases to the first 
quarter, at the greatest western elongation. When 
passing through the superior conjunction, the other side 

262. How does an inferior planet appear when at its greatest 
elongation 1 How when between that and the inferior conjunction? 
How toward the superior conjunction 1 In what respects do they 


of the sun, the enlightened part constantly increases, 
and becomes like the full moon in the superior con- 
junction, after which the enlightened portion decreases. 
The phases of Mercury and Venus, therefore, as seen 
in the telescope, resemble the changes of the moon. 
In some respects, however, the appearances do not 
correspond to those of the moon ; for since, when full, 
they are in the part of the orbit most remote from us, 
they appear then much smaller than when on the side 
of the inferior conjunction ; and their nearness to the 
sun, when full, also prevents their being seen except 
in the day time, and then they are invisible to the naked 
eye, because their light is lost in that of the sun. 
Hence, these planets appear brightest when a little lessj 
than half their enlightened sides are turned toward 

us, (being then just within their greatest elongation on^ 
either side,) since their greater nearness to us morel 
than compensates for having in view a less portion of; 
the enlightened disk, as will be seen by the acconv 
panying diagram. 

263. Mercury and Venus both revolve on their axes 
in nearly the same time with the earth, and have 
therefore similar days and nights. Mercury owes-' 

resemble the changes of the moon 1 How do they differ 1 At what 
point do the inierior planets appear brightest 1 


almost all its peculiarities to its nearness to the sun. 
Its light and heat derived from the sun are estimated 
to be nearly seven times as great as ours, and the sun 
would appear to an inhabitant of Mercury seven times 
as large as it does to us. The motion of Mercury, in his 
revolution round the sun, is swifter than that of any 
other planet, being more than 100,000 miles every 
hour ; whereas, that of the Earth is less than 70,000. 
Eighteen hundred miles every minute crossing the 
Atlantic ocean in less than two minutes this is a ve- 
locity of which we can form but very inadequate con- 

264. Every time Mercury and Venus come to their 
inferior conjunction, they would eclipse the sun, if 
their orbits coincided with the earth's orbit, or both 
were in the same plane ; as we should have a solar 
eclipse at every new moon, if the moon's orbit were 
in the same plane with the earth's. As, however, the 
orbits of these planets are inclined to the ecliptic, they 
are not seen on the sun's disk except when the con- 
junction takes place at one of their nodes. They then 
pass over the sun, each in a round black spot, and the 
phenomenon is called a Transit. Transits of Mer- 
cury and Venus occur but seldom, but are regarded 
with the highest interest by astronomers, that of Ve- 
nus, in particular ; for, by observing it at distant points 
on the earth, materials are obtained for finding the 
sun's horizontal parallax, which enables astronomers 
to calculate the distance of the sun from the earth. 
(See Art. 219.) In the transits of Venus, in 1761 
and 1769, several European governments fitted out 
expensive expeditions to parts of the earth remote 
from each other. For this purpose, the celebrated 

263. In what time do Mercury and Venus revolve on their axesl 
To what does Mercury owe its peculiarities 1 Explain his swiftness 
of motion. 

264. Why do not Mercury and Venus eclipse the sun at every in- 
ferior conjunction 1 What is a transit 1 Why regarded with so great 




Captain Cook, in 1769, went to the South Pacific 
Ocean, and observed the transit of Venus at the island 
of Otaheite, (Tahiti,) while others went to Lapland 
for the same purpose, and others, still, to many other 
parts of the globe. The next transit of Venus will 
happen in 1874. 

SEC. 3. Of the Superior Planets. 

265. All .the planets, except Mercury and Venus, 
have their orbits farther from the sun than the earth's 
orbit. They are seen in superior conjunction with 
the sun, and in opposition, like the moon when full ; 
but as they are always more distant from the sun than 

the earth is, they can never come into inferior con- 
junction. This will be plain from the foregoing dia- 

interest 1 What is said of the transits of Venus in 1761 and 176ft 
When will the next transit of Venus happen 1 


gram. Let the Earth be at E, and a superior planet, 
as Mars, in different parts of his orbit, M Q, M'. At 
M', the planet would be seen in the same part of the 
heavens with the sun, rising and setting at the same 
time with him, and would therefore be in conjunction; 
but being the other side of the sun, it would, of course, 
be a superior conjunction. At Q, the planet would ap- 
pear in quadrature, and at M, in opposition, rising when 
the sun sets, like the full moon. 

266. The superior planets, however, do not, like the 
inferior, undergo the same changes as the moon, but, 
with the exception of Mars, always present to the 
telescope their disks fully enlightened ; for, if we 
viewed them from the sun, we should have the whole 
enlightened side turned constantly toward us; and 
so small is our own distance from the sun, compared 
with that of Jupiter, Saturn, or Uranus, that we view 
them nearly as though we stood on the sun. Mars, 
being nearer the earth, does in fact change his figure 
slightly ; for, when seen in quadrature, at Q, a small 
part of the enlightened hemisphere is concealed from 
us, and the planet appears gibbous, like the moon 
when a little past the full. The superior planets, 
however, undergo considerable changes in apparent 
magnitude and brightness, being at one time much 
nearer to us than at another. Thus, in Fig. Ill, 
Mars, when at M, in opposition, is nearer the Earth 
than at M', in superior conjunction, by the whole 
diameter of the earth's orbit a space of about 
190,000,000 miles. Hence, when this planet is in 
opposition, rising soon after the sun sets, it often sur- 
prises us by its unusual splendor, which appears more 

265. What are superior planets'? How do they differ from the 
inferior ? Explain their conjunction and opposition by the tigure. 

266. Have the superior planets any phases 1 What is said of the 
phases of Mars 1 what changes of apparent magnitude do the su- 
perior planets undergo 1 Explain the cause of these. 


striking on account of its fiery red color. All the other 
planets, likewise, appear finest when in opposition, al- 
though the remoter planets are less altered than those 
that are nearer to us. 

267. JUPITER is distinguished from all the other 
planets by his great magnitude. His diameter is 
89,000 miles, and his volume 1281 times that of the 
earth. He revolves on his axis once in about ten 
hours, giving to places near his equator a motion 


twenty-seven times as swift as on the earth. It will 
be recollected, also, that the distance of Jupiter from 
the sun is 485,000,000 miles, and that his revolution 
around the sun occupies twelve years ; so that every 
thing belonging to this planet is on a grand scale. 
The view of Jupiter through a good telescope, is one 
of the most splendid and interesting sights in astrono- 
my. The disk expands into a large and bright orb, 
like the full moon ; across the disk, arranged in paral- 
lel stripes, are several dusky bands, called belts ; and 

267. By what is Jupiter distinguished from all the other planets'' 

Lis diameter volume distance from the sun 1 View of Jupiter 

through a good telescope 1 Appearance of his disk, belts, and sat 


four bright satellites, or moons, constantly varying 
their positions, add another feature of peculiar mag- 

268, SATURN has also within itself a system full of 
grandeur. Next to Jupiter, it is the largest of the 
planets, being 79,000 miles in diameter, or about 1000 
times as large as the earth. It has, likewise, belts on 
its surface, though less distinct than those of Jupiter. 

Fig. 113. 

But the great peculiarity of Saturn is its Ring, a broad 
wheel, encompassing the planet at a great distance from 
it. What appears to be a single ring, when viewed 
with a small telescope, is found, when examined by 
powerful telescopes, to consist of two rings, separated 
from each other by a dark line of the sky, seen between 
them. Although the division of the rings appears to 
us, on account of our immense distance, as only a fine 
line, yet it is in reality an interval of not less than 
1,800 miles ; and, although we see in the telescope 
but a. small speck of sky between the planet and the 
ring, yet it is really a space 20,000 miles broad. The 

268. Saturn compared with Jupiter diameter vplume belts 
Ring-* what is said of this 1 Distance between the rings. Breadth 



breadth of the inner wheel is 17,000 miles, and that 
of the outer, 10,500 miles ; so that the entire diameter 
of the outer ring, from outside to outside, is 179,000 
miles. These rings are so far from the body of the 
planet, that an inhabitant of that world would not take 
them for appendages to his own planet, but would view 
them as magnificent arches on the face of the starry 

Fig. 114. 

269. Saturn's ring, in its revolution with the planet 
around the sun once in about thirty years, always keeps 
parallel to itself, as is represented in the annexed 
diagram, where the small circle, a Z>, is the earth's 
orbit, and Saturn is exhibited in eight different positions 
in his orbit. If we hold a circle, as a piece of coin, 
directly before the eye, we see the entire circle ; but 
if we hold it obliquely, it appears an ellipse ; and if 
we turn it round until we see it edgewise, the ellipse 
grows constantly narrower and narrower, until, when 
the edge is toward us, we see nothing but a line. If 

of each wheel. Entire diameter of the outer ring. What is said 
of the appearance of the rings from the planet 1 

269. What position does the ring keep in its revolution around the 
sun 1 Describe Fig 114. Into what figures is a circle projected 


the learner obtains a clear idea of these appearances, 
he will easily understand the different appearances of 
Saturn's ring. In two points of the revolution around 
the sun, at A and E, the edge is presented to us, and 
we see the ring only as a fine line, or, perhaps, lose 
sight of it altogether. After passing this point, from 
B to C, we see more and more of the ellipse, until, in 
about seven years, it arrives at C, when it appears quite 
broad, as represented in figure 114. Then it gradually 
closes again for seven years more, and dwindles into a 
line at E. 

270. Saturn is attended by seven satellites. Al- 
though they are bodies of considerable size, yet, on 
account of their immense distance from us, they appear 
exceedingly minute, and require superior telescopes 
to see them at all. It is accounted a good telescope 
which will give a distinct view of even three of the 
satellites of Saturn, and the whole seven can be seen 
only by the most powerful telescopes in the world. 

271. URANUS is also a large body, being 35,000 
miles in diameter ; but being 1800,000,000 miles off, 
it is scarcely seen except by the telescope, and would 
hardly be distinguished from a fixed star, if it were 
not seen to have the motions of a planet. In the most 
powerful telescopes, however, it exhibits more of the 
character of a planet. Herschel saw, as he supposed, 
six satellites belonging to this planet, but only two are 
commonly visible to the best telescopes. So distant is 
this planet, that the sun himself would appear from it 
400 times less than he does to us, and it receives from 
him light and heat proportionally feeble. 

when seen in different positions ^ In what points is the edge pre- 
sented to us 1 When does it appear broadest 1 

270. How many satellites has Saturn 1 How do they appear to 
the telescope 1 What power does it require to see them 1 

271. Uranus his diameter distance from the sun appearance 
in the telescope number of satellites. How would the sun appear 
from Uranus ] 


272. The NEtv PLANETS, or ASTEROIDS, Ceres, 
Pallas, Juno, and Vesta, were unknown until the 
commencement of the present century. They are so 
small as to be invisible to the naked eye, but are seen by 
telescopes of moderate power. They lie near together 
in the large space between the orbits of Mars and 
Jupiter, at an average distance from the sun of about 
250,000,000 miles. 

SEC. 4. Of the Planetary Motions. 

273. The planets all revolve around the sun in the 
same direction, from west to east, and pursue nearly 
the same path in the heavens. Mercury wanders 
farthest from the general track, but he is never seen 
farther than about seven degrees from the ecliptic. The 
others, with the exception of the Asteroids, are always 
seen close in the neighborhood of the ecliptic, and we 
never need to look in any other part of the sky for a 
planet, than in the region of the sun's apparent path in 
the heavens. 

274. If we could stand on the sun and view the* 
planets move round it, their motions would appear 
very simple. We should see them, one after another, 
pursuing their way along the great highway of the 
heavens, the zodiac, rolling around the sun as the 
moon does around the earth, though with very different 
degrees of speed, those near the sun moving with 
far more rapidity than those more remote, often over- 
taking them, and passing rapidly by them. Mercury, 
especially, comes up with and passes Jupiter, Saturn, 
atid Uranus, a great number of times while they are 

272. What is said of the New Planets their discovery size 
position in the solar system distance from the sum 1 

273. Pknetary motions through what part of the heavens which 
wanders farthest from the ecliptic 1 

274. If we could view the planets from the sun, how would they 
appear to move 1 In what orbits, and with what difierciit degrees 
oi speed 1 


making their tardy circuit around the sun. To a spec- 
tator thus situated, the planets would all appear to 
move around him in great circles, such being their 
projections on the face of the sky. They are, how- 
ever, not perfect circles, but are a little shorter in one 
direction than the other, forming an oval or ellipse. 

275. Such would be the appearances of the planet- 
ary motions if viewed from the center of their motions, 
that is, at the sun, and such they are in fact. But two 
causes operate to make the motions of the planets ap- 
pear very different from what they really are ; first, we 
view them out of the center of their motions, and, sec- 
ondly, we are ourselves in motion. We have seen, 
in the case of the inferior planets, Mercury and Ve- 
nus, that our being out of the center makes them ap- 
pear to run backward and forward across the sun, 
although they are all the while moving steadily on in 
one direction; and we know that our own motion 
along with the earth on its axis, every day, makes the 
heavens appear to move in the opposite direction. 
Hence, we see how very different may be the actual 
motions of the planets from what they appear to be. 
As we have said, they are actually very simple, mov- 
ing steadily round the sun, all in one direction ; but 
their apparent motions are exceedingly irregular. 
They sometimes move faster and sometimes slower 
backward and forward and at times appear to stand 
still for a considerable period. 

276. If we have ever passed swiftly by a small ves- 
sel, sailing in the same direction with ourselves, but 
much slower, we may have seen the vessel appear to 
be moving backward, stern foremost. For a similar 
reason, the superior planets sometimes seem to move 
backward, merely because the earth has a swifter 

275. What makes the planetary motions appear very different 
from what they really are 1 Are the real motions more or less sim- 
ple than the apparent 1 



motion, and sails rapidly by them. Then again they 
seem to stand still, because they are about turning, 
when our motion has ceased to carry them apparently 
backward any farther, and they are recovering their 
direct motion. They appear also to stand still, when 
they are moving directly toward us or from us, as 
Mercury or Venus does when near its greatest elonga- 
tion. (See Fig. 109, page 233.) A diagram will as. 
sist us in obtaining a clear idea of the way in which 
these appearances are produced. 

Fig. 115. 

277. Let the inner circle, ABC, represent the 
earth's orbit, and the outer circle the orbit, of Mars, 

276. Appearance of a vessel when we pass rapidly by it 1 Why d 
the superior planets appear to move backward, and to stand still 1 


(or any other superior planet,) and N R a portion of 
the concave sphere of the heavens. To make the 
case simple, we will suppose Mars to be stationary at 
M, in opposition ; for, although he is actually moving 
eastward all the while, yet, since the earth is moving 
the same way more rapidly, their relative situations 
will be the same, if we suppose Mars to stand still and 
the earth to move on with the excess of its motion 
above that of the planet. As the earth moves from A 
to B, Mars appears to move backward from P to N ; 
for the planet will always appear in the heavens in the 
direction of the straight line, as B M, drawn from the 
spectator to the body. When the earth is at B, Mars 
appears stationary, because the earth is moving directly 
from him, and the line B M N does not change its di- 
rection. But while the earth moves on to C, D, E, 
F, the planet resumes a direct motion eastward through 
O, F, Q, R. Here it again stands still, while the 
earth is moving directly toward it, and then goes back- 
ward again. When the planet is in opposition, the 
earth being at A, its motion appears more rapid than 
in other situations, because then it is nearest to us. 
In the superior conjunction, when the earth is at D, 
the motion of Mars is comparatively slow. 

278. There are three great Laws that regulate the 
motions of all bodies belonging to the Solar System, 
called KEPLER'S Laws, from the name of the great as- 
tronomer who discovered them. The first is, that the 
orbits of the earth and all the planets are ellipses, having 
the sun in one of the foci of the ellipse. Figure 116 re- 
presents such an ellipse, differing but little from a cir- 
cle, but still having the diameter, A B, called the major 
axis of the orbit, perceptibly longer than C D. The 

277. Illustrate the motion of Mars from Fig,U. 
tion most rapid 1 When slow 1 

278. Kepler's Laws. Repeat the first law. What is an eUipsfc 
the major axis foci perihelion aphelion 1 


two points, E and F, (being the points from which, by 
a certain process, the figure is described,) are called 

the two foci, and each of them, a focus, of the ellipse. 
Suppose the sun at F, then B will be the perihelion or 
nearest distance of a planet to the sun, and A is the 
aphelion, or farthest distance. 

279. A line drawn from the sun to a planet is called 
the radius vector, as E a or E #, (Fig. 117 ;) and the 
second of Kepler's Laws is, that while a planet is going 
round the sun, the radius vector passes over equal spaces 
in equal times. The meaning of this is, that, if an 
imaginary line, as a cord, were extended from the sun 
to any planet, this cord would sweep over just as much 
space one day as another. When the planet is at its 
perihelion, the cord would, indeed, move faster than 
toward the aphelion ; but it would also be shorter, 

279. What is the radius vector 1 Repeat the second law. 
plain its meaning. 




and the greater breadth of the space, E a J, would 
make it just equal to the narrower but longer space, 

Fig. 117. 

E I m. This law has been of incalculable service in 
all the higher investigations of astronomy. 

280. The third of Kepler's Laws is, that the squares 
of the periodic times of different planets, are proportioned 
to the cubes of the major axes of their orbits. Now the 
periodic time of a planet, or the time it takes to go round 
the sun, from any star back to the same star again, can 
be seen by watching it, as has often been done, during 
the whole of its revolution. We also know the length 
of the major axis of the earth's orbit, because it is just 
twice the average or mean distance of the earth from 
the sun. These things being known, we can find the 
distance of any of the planets from the sun by a simple 
statement in the rule of three. For example, let it be 
required to find the major axis of Jupiter's orbit, or the 

280. Repeat the third law. What is meant by the periodic time 
of a planet 1 How may the periodic time be found J Do we know 


mean distance of Jupiter from the sun, which is half 
the length of that axis. Then, since the earth's 
periodic time is one year, and Jupiter's twelve years, 
(putting E for the earth's distance from the sun, and J 
for Jupiter's,) we say, 

I 2 : 12 2 : : E 3 : J 3 . 

Now the three first terms in this proportion are 
known, and hence we can find the fourth, which is 
the cube of Jupiter's distance from the sun ; and, on 
extracting the cube root, we find the distance itself. 
We see, therefore, that the planetary system is laid off 
by an exact mathematical scale. 

281. The three foregoing laws are so many great 
facts, fully entitled to be called general principles, 
because they are applicable not only to this or that 
planet, but to all the planets alike, and even to comets, 
and every other kind of body that may chance to be 
discovered in the solar system. They are the rules 
according to which all the motions of the system are 
performed. But there is a still higher inquiry, respect- 
ing the causes of the planetary motions, which aims at 
ascertaining not in what manner the planets move, but 
why they move at all, and by what forces their motions 
are produced and sustained. Sir Isaac Newton first 
discovered the great principle upon which all the 
motions of the heavenly bodies depend, that of Universal 
Gravitation. In its simplest expression it is nearly this : 
all matter attracts all other matter. But a more precise 
expression of the law of gravitation is as follows : 
Every body in the universe, whether great or small, attracts 
every other body, with a force which is proportioned to 

the major axis of the earth's orbit 1 How to find the major axis oi 
Jupiter's orbit 1 

281. Why are these laws called general principles 1 What higher 
inquiry is there 1 Who first discovered the grand law of the celes- 
tial motions'? What is it called 1 Its simplest expression. Ita 
more precise expression 


the quantity of matter directly, and to the square of the 
distance inversely. 

282. This is the most comprehensive and important 
of all the laws of nature, and ought therefore to be 
clearly understood in. its several parts. First, it asserts 
that all matter in the universe is subject to it. In this 
respect it differs from Gravity, which respects only the 
attraction exerted by the earth, and all bodies within 
the sphere of its influence. But Universal Gravitation 
embraces the whole solar system sun, moon, planets, 
comets, and any other form of matter within the 
system. Nor does it stop here ; it extends likewise to 
the stars, and comprehends the infinitude of worlds 
that lie in boundless space. Secondly, the law asserts 
that the attraction of gravitation is in proportion to the 
quantity of matter. Every body gives and receives of this 
mysterious influence an amount exactly proportioned 
to its weight ; and hence all bodies exert an equal 
force on each other. The sun attracts the earth and 
the earth the sun, and one just as much as the other ; 
for if the sun, in consequence of its having 354,000 
times as much matter as the earth, exerts upon it 
354,000 times as much force as it would do if it had 
the same weight with the earth, it also receives from 
the earth so much more in consequence of its greater 
weight. Were the sun divided into 354,000 bodies, 
each as heavy as the earth, every one would receive 
an equal share of the earth's attraction, and of 
course the whole would receive in the same degree 
as they imparted. Thirdly, the law asserts that, at 
different distances, the force of gravitation is in- 
versely as the square of the distance. If a body 
is twice as far off, it attracts and is attracted four 

282. What is said of the importance of this law 1 What does it 
assert first what secondly ? How much does every body give and 
receive of this influence 7 Example in the earth and sun. What 
does the law assert thirdly ? How much less does a body attract 
another when twice as far off, or ten times as far 1 


times less ; if ten times as far, one hundred times 
less ; if a hundred times as far, ten thousand times 

283. This great principle, which has led to a 
knowledge of the causes of the celestial motions, and 
given us an insight into the machinery of the Universe, 
was discovered by Sir Isaac Newton, who is generally 
acknowledged to have had the most profound mind of 
any philosopher that has ever lived. He was born in 
a country town in England in the year 1642. He was 
a farmer's son, and his father having died before he 
was born, his friends designed him for a farmer ; but 
his strong and unconquerable passion for study, and the 
great mechanical genius he displayed in his boyhood, 
led them to the fortunate determination to educate him 
at the University. 

284. But let us see how the principle of Gravitation 
is applied to explain the revolutions of the heavenly 
bodies. If I throw a stone horizontally, the attraction 
of the earth will continually draw it downwards, out of 
the line of direction in which it was thrown, and make 
it descend to the earth in a curve. The particular 
form of the curve will depend on the velocity with 
which it is thrown. It will always "begin to move in 
the line of direction in which it is projected ; but it will 
soon be turned from that line toward the earth. It will, 
however, continue nearer to the line of projection, 
in proportion as the velocity of projection is greater. 
Let A C (Fig. 118) be perpendicular to the hori- 
zon, and A B parallel to it, and let a stone be 
thrown from A in the direction of A B. It will, in 
every case, commence its motion in the line A B, 
which will therefore be a tangent to the curve it de- 

283. What is said of Sir Isaac Newton 1 

284. How is the principle of universal gravitation applied to the 
explanation of the celestial motions 1 How will a stone move 
when thrown horizontally 1 Explain Fig. 118. 



scribes ; but, if it be thrown with a small velocity, it 
will soon depart from the tangent, describing the curve 

Fig. 118. 

A D ; with a greater velocity, it will describe a curve 
nearer the tangent, at A E ; and with a still greater 
velocity, it will describe the curve A F. 

285. As an example of a body revolving in an 
orbit under the influence of two forces, suppose a body 

Fig. 119. 

placed at any point, P, (Fig. 119,) above the surface 
of the earth, and let P A be the direction of the earth's 

285. Explain the motions of a body from Fig. 119. 



center, or a line perpendicular to the horizon. If the 
body were allowed to move, without receiving any 
impulse, it would descend to the earth in the direction 
of P A with an accelerated motion. But suppose that 
at the moment of its departure from P, it receives a 
blow in the direction P B, which would carry it to B ! i 
in the time the body would fall from P to A ; then i 
under the influence of both forces, it would descend 
along the curve P D. If a stronger blow were given 
to it in the direction P B, it would describe a larger 
curve, P E ; or, finally, if the impulse were sufficiently \ 
strong, it would circulate quite round the earth, de- i 
scribing the circle P F G. With a velocity of projec- 
tion still greater, it would describe an ellipse, P I K ; 
and if the velocity were increased to a certain degree, 
the figure would become a parabola, L P M, a curve 
which never returns into itself. 

286. Now let us con- 
sider the same princi- 
ples in reference to the 
motion of a planet around 
the sun. Suppose the 
planet to have passed 
\Kthe point C, (Fig. 120,); 
at the aphelion, with so;j 
small a velocity, that the 
attraction of the sunj 
bends its path toward' 
itself. As the body ap- ' 
proaches the sun, since 
the sun's attractive force 
is rapidly increased as 
the distance is dimin- 1 
ished, the planet's motion is continually accelerated,] 
and becomes very swift as it approaches nearer the sun. j 
But, when a body is revolving in a curve, an increase of 1 

286. Explain the motions of a planet from Fig. 120. 


velocity causes a rapid increase in the centrifugal force, 
and makes it endeavor with more and more force to fly 
off in the direction of a tangent to its orbit. Hence, the 
increase of velocity as it approaches the sun, will not 
carry it into the sun, but the more rapid increase of 
the centrifugal force will keep it off, and carry it by, 
and finally make it describe the remaining portion of 
the curve, back to the place where it set out. After 
it passes the perihelion, at G, the sun's attraction con- 
stantly operates to hold it back, and as it proceeds 
through H and K to A and C, it is like a, ball rolled 
up hill, until finally its motion becomes so slow, that 
the centrifugal force yields to the force of attraction, 
and it turns about to renew the same circuit. 

287. Since the nature of the curve which any planet 
describes depends on the proportion between the two 
forces, of projection and attraction, astronomers have 
inquired what proportion must have been observed 
when the planets were first launched into space, in 
order that they should have revolved in the orbits they 
have ; and it is found that the forces were so adjusted 
as to make the centrifugal and attractive forces nearly 
equal, that of projection being a little greater. Had 
they been exactly equal, the curve would have been a 
circle ; and had the force of projection been much 
greater than it was, the ellipses would have been much 
longer, and the whole system much more irregular. 
The planets also revolve on their axes at the same time 
that they revolve around the sun ; and astronomers 
have inquired what must have been the nature of the 
impulses originally given, in order to have produced 
these two motions such as they are. If we strike a 
ball in the exact line of the center of gravity, it will 
move forward without turning on its axis ; but if we 

287. How were the forces of projection and attraction adjusted to 
each other, when the planets were first launched into space 1 How 


strike it out of that direction we can make it move for- 
ward and turn on its axis at the same time. It is cal- 
culated that the earth must have received the impulse 
which gave to her her two motions, at a distance from 
the center equal to the T |^th part of the earth's radius. 
Such an impulse would suffice to give the two motions 
in question ; but it would be presumptuous to under- 
take to assign the exact mode by which the Almighty 
first impressed upon the planetary system its harmoni- 
ous movements; and all such expressions as " launch- 
ing these bodies into space," or " impelling" them in 
certain directions, must be regarded as mere figures of 

288. Besides explaining the revolutions of the hea- 
venly bodies, the principle of universal gravitation ac- 
counts for all their irregularities. Since every body 
in the solar system attracts every other, each is liable 
to be drawn out of its customary path, and all the 
bodies tend mutually to disturb each other's motions. 
Most of them are so far apart as to feel each other's 
influence but little ; but in other cases, where any two 
bodies come far within each other's sphere of attrac- 
tion, the mutual disturbance of their motions is very 
great. The moon, especially, has its motions con- 
tinually disturbed by the attractive force of the sun. 
When the sun acts equally on the earth and the moon, 
as it does when the two bodies are at the same dis- 
tance from him, he does not disturb their mutual rela- 
tions ; as the passengers on board a steamboat main- 
tain the same position with respect to each other, 
whether the boat is going with or against the current. 
But, at new moon, the moon being nearer the sun than 

must they have been impelled in order to have the two motions 1 
How must the earth have been struck 1 

288 Besides the revolutions of the heavenly bodies, for what else 
does the principle of universal gravitation account ^ How does the at- 
traction of different bodies tend to affect each other's motions ? What 
is said of the moon 1 When does the sun disturb the mutual relations 


the earth is, is more attracted than the earth ; and at 
full moon, the earth being nearer the sun than the moon 
is, is more attracted than the moon. Hence, in both 
cases, the sun tends to separate the two bodies. At 
other times, as when the rnoon is in quadrature, the 
influence of the sun tends to bring the bodies nearer to- 
gether. Sometimes it causes the moon to move faster, 
and sometimes slower ; so that owing to these various 
causes, the moon's motions are continually disturbed, 
which subjects her to so many irregularities, that it 
has required vast labor and research to ascertain the 
exact amount of each, and so to apply it as to assign 
the precise place of the moon in the heavens at any 
given time. 

289. Among all the irregularities to which the 
heavenly bodies are subject, there is not one which the 
principle of universal gravitation does not account for, 
and even render necessary ; so that if it had never 
been actually observed, a just consideration of the con- 
sequences of the operation of this principle, would 
authorize us to say that it must take place. Indeed, 
many of the known irregularities were first discovered 
by the aid of the doctrine of gravitation, and afterward 
verified by actual observation. Such a tendency of 
all the heavenly bodies to disturb each other's motions, 
might seem to threaten the safety of the whole system, 
and throw the whole into final disorder and ruin ; but 
astronomers have shown, by the aid of this same prin- 
ciple, that all possible irregularities which can occur 
among the planets, have a narrow, definite limit in- 
creasing first on one side, then on the other, and thus 

of the moon and earth 1 When does the sun attract the moon more 
than the earth7 When the. earth more than the moon 1 What va- 
rious disturbances does it produce on the moon's motions 1 

289. Does the principle of universal gravitation account for the 
irregularities of the celestial motions 7 How were many of them 
first discovered 1 Will these irregularities produce disorder and 
ruin 1 What has been shown respecting their limit 1 


vibrating for ever about a mean value, which secures 
the stability of the universe. 




290. NOTHING in astronomy is more truly admirable, 
than the knowledge which astronomers have acquired 
of the motions of comets, and the power they have 
gained of predicting their return. Indeed, everything 
belonging to this class of bodies is so wonderful, as to 
seem rather a tale of romance than a simple recital of 

291. A comet, when perfectly formed, consists of 
three parts, the nucleus, the envelope, and the tail. 
The nucleus, or body of the comet, is usually distin- 
guished by its forming a bright point in the center of 
the head, conveying the idea of a solid, or at least of 
a dense portion of matter. Though it is usually very 
small when compared with the other parts of the comet, 
and is sometimes wanting altogether, yet it occasion- 
ally is large enough to be measured by the aid of the 
telescope. The envelope (sometimes called the co-ma, 
from a Latin word signifying hair, in allusion to its 
hairy appearance,) is a thick, misty covering, that sur- 
rounds the head of the comet. Many comets have nqr 
nucleus, but present only a foggy mass. Indeed, there 
is a regular gradation of comets, from such as are com- 
posed merely of a gaseous or vapory medium, to those 

290. What is said of the knowledge astronomers have gained 
of comets 1 

291. Specify the several parts of a comet, and describe each part 

COMETS. 263 

which have a well-defined nucleus. In some instances, 
astronomers have detected, with their telescopes, small 
stars through the densest part of the comet. The tail 
is regarded as an expansion or prolongation of the en- 
velope, and presenting, as it sometimes does, a train 
of astonishing length, it confers on this class of bodies 
their peculiar celebrity. These several parts are ex- 
hibited in Fig. 121, which represents the appearance 

Fig. 121. 

of the celebrated comet of 1680, and which, in general 
size and shape, is not unlike that of 1843. The latter, 
however, was not so broad in proportion to its length, 
and its head (including the nucleus and coma) was far 
less conspicuous. 

292. In magnitude and brightness, comets exhibit 
great diversity. History informs us of several comets 
so bright as to be distinctly visible in the daytime, 
even at noon, and in the brightest sunshine. Such 
was the comet seen at Rome a little before the assas- 

the nucleus the envelope the tail. How did the comet of 1680 
compare with that of 18431 

292. What is said of the magnitude and brightness of comets 7 
Of the comet seen at Rome 1 Of that of 16SO 1 Of 1811 1 How 


sination of Julius Caesar ; and, in a superstitious age, 
very naturally considered as the precursor of that event. 
The comet of 1680 covered an arc of the heavens of 
ninety-seven degrees, sufficient to reach from the set- 
ting sun to the zenith, and its length was estimated at 
123,000,000 miles. The comet of 1811 had a nucleus 
only 428 miles in diameter, but a tail 132,000,000 
miles long ; and had it been coiled around the earth 
like a serpent, it would have reached round more than 
5000 times. Other comets are exceedingly small, the 
nucleus being in one case estimated at only 25 miles ; 
and some which are destitute of any perceptible nu- 
cleus, appear to the largest telescopes, even when near- 
est to us, only as a small speck of fog. The majority 
of comets can be seen only by the aid of the telescope. 
Indeed, the same comet has different appearances at 
its different returns. Halley's comet, in 1305, was 
described by the historians of that age as the comet of 
" horrific magnitude ;" yet, in 1835, when it reap- 
peared, the greatest length of its tail was only about 
twelve degrees, whereas that of the comet of 1843 was 
about forty degrees. 

293. The periods of comets, in their revolutions 
around the sun, are equally various. Encke's comet, 
which has the shortest known period, completes its 
revolution in 3-J- years ; while that of 1811 is estimated 
to have a period of 3,383 years. The distances to 
which different comets recede from the sun are equally 
various. While Encke's comet performs its entire 
revolution within the orbit of Jupiter, Halley's comet 
recedes from the sun to twice the distance of Uranus, 
or 3600,000,000 miles. Some comets, indeed, are 
thought to go a much greater distance from the sun 

small are some comets 1 How does the same comet appear at its 
different returns 1 

293. What is said of the periods of the comets'? Of Encke's 
comet 1 Of that of 1811 1 What of the distances to which they 
recede from the sun 1 

COMETS. 265 

than this; while some are supposed to pass into curves, 
which do not, like the ellipse, return into themselves ; 
and, in this case, they never come back to the sun. 

294. Comets shine by reflecting the light of the sun. 
In one or two cases, they have been thought to exhibit 
distinct phases, like the moon, and experiments made 
on the light itself, indicate that it is reflected and not 
direct light. The tails of comets extend in a direct 
line from the sun, following the body as it approaches 
that luminary, and preceding the body as it recedes 
from it. 

295. The quantity of matter in comets is exceedingly 
small. The tails consist of matter so light, that the 
smallest stars are visible through them. They can 
only be regarded as masses of thin vapor, susceptible 
of being penetrated through their whole substance by 
the sunbeams, and reflecting them alike from their 
interior parts and from their surfaces. " The highest 
clouds that float in our atmosphere,'*' (says a great 
astronomer, Sir John Herschel,) " must be looked upon 
as dense and massive bodies compared with the filmy 
and all but spiritual texture of a comet." The small 
quantity of matter in comets is proved by the fact, 
that they have at times passed very near to some of 
the planets, without disturbing their motions in any 
appreciable degree. As the force of gravity is always 
proportioned to the quantity of matter, were the density 
of these bodies at all comparable to their size, on 
coming near one of the planets, they would raise 
enormous tides, and perhaps even draw the planet 
itself out of its orbit. But the comet of 1770, in its 
way to the sun, got entangled among the satellites of 
Jupiter, and remained near them four months ; yet it 

294. By what light do comets shine 1 Do they ever exhibit phases 1 
What is the direction of their tails 1 

295. Quantity of matter in comets'? Extreme thinness 1 ? What 
proofe are stated to show their small quantity of matter 1 What is 



did not perceptibly change their motions. The same 
comet also came very near to the earth ; so that, had 
its quantity of matter been equal to that of the earth, 
it would, by its attraction, have caused the earth to have 
revolved in an orbit so much larger than at present, as 
to have increased the length of the year two hours and 
forty-seven minutes. Yet it produced no sensible effect 
on the length of the year. It may, indeed, be asked, 
what proof we have that comets have any matter, and 
are not mere reflexions of light ? The answer is, 
that although they are not able, by their own force of 
attraction, to disturb the motions of the planets, yet 
they are themselves exceedingly disturbed by the action 
of the planets, and in exact conformity with the laws 
of universal gravitation. A delicate compass may be 
greatly agitated by the vicinity of a mass of iron, while 
the iron is not sensibly affected by the attraction of the 

296. The motions of comets are the most wonderful 
of all their phenomena. When they first come into 
view, at a great distance from the sun, as is sometimes 
the case, they make very slow approaches from day to 
day, and even, in some cases, advance but little from 
week to week. When, however, they come near to the 
sun, their velocity increases with prodigious rapidity, 
sometimes exceeding a million of miles an hour ; they 
wheel around the sun like lightning ; and recede again 
with a velocity which diminishes at the same rate as 
it before increased. We have seen that the planets 
move in orbits which are nearly circular, and that 
therefore they always keep at nearly the same distance 
from the sun. Not so with comets. Their perihelion 
distance is sometimes so small that they almost graze 

said of the comet of 1770 1 What proof have we that they contain 
any matter 1 

296. What is said of the motions of comets 1 What is the shape 
of their orbits 1 Of their distance from the sun at the perihelion 

COMETS. 267 

his surface, while their aphelion lies far beyond 
the utmost bounds of the planetary system, towards 
the region of the stars. This was the case with the 
comet of 1680, and the same is probably true of the 
wonderful comet of 1843. But irregular as are their 
motions, they are all performed in exact obedience 
to the great law of universal gravitation. The radius 
vector always passes over equal spaces in equal times ; 
the greater length of the triangular space described 
at the aphelion, where the motion is so slow, being 
compensated by the greater breadth of the triangular 
space swept over at the perihelion, where the motion is 
so swift. 

297. The appearances of the same comet at different 
periods of its return are so various, that we can never 
pronounce a given comet to be the same with one that 
has appeared before, from any peculiarities in its form, 
size, or color, since in all these respects it is very 
different at different returns ; but it is judged to be the 
same if its path through the heavens, as traced among 
the stars, is the same. If, on comparing two comets 
that have appeared at different times, they both moved 
in orbits equally inclined to the ecliptic ; if they crossed 
the ecliptic in the same place among the stars ; if they 
came nearest the sun, or passed their perihelion, in the 
same part of the heavens ; if their distance from the 
sun at that time was the same ; and, finally, if they 
both moved in the same direction with regard to the 
signs, lhat is, both east, or both west ; then we should 
pronounce them to be one and the same comet. But 
if they disagreed in more or less of these particulars, 
we should say that they were not the same but different 

and at their aphelion 1 Are the motions of a comet subject to the 
laws of gravitation 1 

297. How do we determine that a comet is the same with one 
that has appeared before 1 Enumerate the several particulars in 
which the two must agree 1 


298. Having established the identity of a comet with 
one that appeared at some previous period, the interval 
between the two periods would either be the time of its 
revolution, or some multiple or aliquot part of that 
time. Should we, for example, find a present comet 
to be identical with one that appeared 150 years ago, its 
period might be either 150 or 75 years, since possibly 
it might have returned to the sun twice in 150 years, 
although its intermediate return, at the end of 75 years, 
was either not observed or not recorded. Hence the 
method of predicting the return of a comet which has 
once appeared requires, first, that we ascertain with 
all possible accuracy the particulars enumerated in 
article 297, which are called the elements of the comet, 
and then compare these elements with those of other 
comets as recorded in works on this subject. The 
elements of about 130 comets have been found and 
registered in astronomical works, to serve for future 
comparison, but three only have their periodic times 
certainly determined. These are Halley's, Biela's, 
and Encke's comets ; the first of which has a period 
of 75 or 76 years ; the second, of 6| years ; the third, 
of 31 years. 

299. Halley's comet is the most interesting of these, 
and perhaps, on all accounts, the most interesting 
member of the solar system. It was the first whose 
return was predicted with success. Having appeared 
in 1682, Dr. Halley, a great English astronomer, then 
living, ascertained that its elements were the same 
with one that had appeared several times before, at 
intervals corresponding to about seventy-six years, and 
hence pronounced this to be its period, and predicted 

298. When the identity with a previous comet is established, ho\v 
do we learn the time of us revolution 1 What is the method of pre- 
dicting their return 1 Of how many comets have the elements been 
determined 1 How many have their periods certainly ascertained 1 

299. What is said of Halley's comet 1 What prevented Halley'a 
fixing the exact moment of its return 1 What is said about weighing 

COMETS. 269 

that in about seventy-six years more, namely, the lat- 
ter part of 1758 or the beginning of 1759, it would 
return. It did so, and came to its perihelion on the 
13th of March, 1759. What prevented his fixing the 
exact moment, was the uncertainty which then existed 
with respect to the effects of the planets in disturbing 
its motions. Since, in passing down to the sun, it would 
have to cross the orbits of all the planets, and would 
come near to some of them, it was liable thus to be 
greatly retarded in its movements by the powerful at- 
traction of these great bodies. Before the exact amount 
of this force could be estimated, the precise quantity 
of matter in those bodies must be known ; that is, they 
must be weighed. This had been, at that time, imper- 
fectly done. It has since been done with the greatest 
accuracy ; such large bodies as Jupiter and Saturn 
have been weighed as truly and exactly as merchan- 
dise is weighed in scales. Hence, on the late return 
of Halley's comet, in 1835, the precise effect of all 
these disturbing forces was calculated, and the time 
of its return to the perihelion assigned to the very 

300. The success of astronomers in this prediction 
was truly astonishing. During the greatest part of this 
long period of seventy-six years, the body had been 
wholly out of sight, beyond the planetary system, and 
beyond the reach of the largest telescopes. It must 
be followed through all this journey to the distance of 
3600,000,000 of miles from the sun ; and, before the 
precise time of its reappearance could be predicted, 
the amount of all the causes that could disturb its mo- 
tions, arising from the various attractions of the plan- 
ets, must be determined and applied. Since, moreover, 
these forces would vary with every variation of the 

the planets'? How were the predictions respecting Halley's comet 
fulfilled in 18351 

300. What is said of the success of astronomers in this prediction 1 


distance, the calculation was to be made for every de- 
gree of the orbit, separately, through 360 degrees, for 
a period of seventy-six years. Guided, however, by 
such an unerring principle as universal gravitation, 
astronomers felt no doubt that the comet would be true 
to its appointed time, and they therefore told us, months 
beforehand, the time and manner of its first approach, 
and its subsequent progress. They told us that early 
in August, 1835, the comet would appear to the tele- 
scope as a dim speck of fog, at a certain hour of the 
night, in the northeast, not far from the seven stars ; 
that it would slowly approach us, growing brighter and 
larger, until, in about a month, it would become visible 
to the naked eye ; that, on the night of the 7th of Oc- 
tober, it would approach the constellation of the Great 
Bear, and move along the northern sky through the 
seven bright stars of that constellation called the Dip- 
per ; that it would pass the sun about the middle of 
November, and reappear again on the other side of 
the sun about the end of December. All these pre- 
dictions were verified, with a degree of exactness that 
constitutes this one of the highest achievements of 

301. Since comets which approach very near the 
sun, like the comets of 1680 and 1843, cross the or- 
bits of all the planets, in going to the sun and return, 
ing, the possibility that one of them may strike the earth 
has often been suggested, and at times created great 
alarm. It may quiet our apprehensions on this subject 
to reflect on the vast extent of the planetary spaces, 
in which these bodies are not crowded together as we 
see them erroneously represented in orreries and dia- 
grams, but are sparsely scattered at immense distances 
from each other, resembling insects flying in the open 

Describe the difficulties attending it. What did astronomers tell us 
beforehand 1 How were these predictions fulfilled 1 
301. What is said of the danger that a comet will strike the earth 1 


heaven. Such a meeting with the earth is a very im- 
probable event ; and were it to happen, so extremely 
light is the matter of comets, that it would probably 
be stopped by the atmosphere ; and if the matter is 
combustible, as we have some reason to think, it would 
probably be consumed without reaching the earth. 
And, finally, notwithstanding all the evils of which 
comets, in different ages of the world, have been con- 
sidered as the harbingers, we have no reason to think 
that they ever did or ever will do the least injury to 




302. VAST as are the dimensions of the Solar Sys- 
tem, to which our attention has hitherto been confined, 
it is but one among myriads of systems that compose 
the Universe. Every star is a world like this. The 
fixed stars are so called, because, to common observa- 
tion, they always maintain the same situations with re- 
spect to each other. In order to obtain as clear and 
distinct ideas of them as we can, we will consider, un- 
der different heads, the number, classification, and dis- 
tances of the stars their various orders their nature 
and their arrangement in one grand system. 

SEC. 1. Of the Number, Classification, and Distances 
of the Stars. 

What would happen if it should 1 Have comets ever been known 
to do any injury 1 

302. Why are the fixed stars so called 1 Under what different 
heads ,are the fixed stars considered 1 


303. When we look at the firmament on a clear 
winter's night, the number of stars visible even to the 
naked eye, seems immense. But when we actually 
begin to count them, we are surprised to find the num- 
ber so small. In some parts of the heavens, half a 
dozen stars will occupy a large tract of the sky, al- 
though in other parts they are more thickly crowded 
together. Hipparchus of Rhodes, in ancient times, 
first counted the stars, and stated their number at 1022. 
If we stand on the equator, where we can see both the 
northern and southern hemispheres, and carefully enu- 
merate the stars that come into view at all seasons of 
the year, the entire number will amount to 3000. The 
telescope, however, brings to view hosts of stars in- 
visible to the naked eye, the number increasing with 
every increase of power in the instrument ; so that we 
may pronounce the number of stars that are actually 
distributed through the fields of space, to be literally 
endless. Single groups of half a dozen stars, as seen 
by the naked eye, often appear to a powerful telescope 
in the midst of hundreds of others of feebler light. 
Astronomers have actually registered the positions of 
no less than 50,000 ; and the whole number visible in 
the largest telescopes amounts to many millions. 

304. The stars are classed by their apparent mag- 
nitudes. The whole number of magnitudes recorded 
is sixteen, of which the first six only are visible to the 
naked eye ; the rest are telescopic stars. These mag- 
nitudes are not determined by any very definite scale, 
but are merely ranked according to their relative de- 
grees of brightness, and this is left in a great measure 
to the judgment of the eye alone. The brightest stars, 

303. Apparent number of the stars on a general view. Result 
when we count them. Who first made a catalogue of the stars 1 How 
many were included * What is the greatest number visible to the 
naked eye 1 Numbers visible in the telescope 1 Whole number 1 

304. How are the stars classed 1 How many magnitudes *? How 
many of them are visible to the naked eye 1 What are the rest called I 


to the number of fifteen or twenty, are considered as 
stars of the first magnitude ; the fifty or sixty next 
brightest, of the second magnitude ; the next two 
hundred, of the third magnitude; and thus the number 
of each class increases rapidly, as we descend the 
scale, so that no less than fifteen or twenty thousand 
are included within the first seven magnitudes. 

305. The stars have been grouped in constellations 
from the most remote antiquity. A few, as Orion, 
Bootes, and Ursa Major, (the Great Bear,) are men- 
tioned in the most ancient writings, under the same 
names as they have at present. The names of the 
constellations are sometimes founded on a supposed 
resemblance to the objects to which those names be- 
long ; as the Swan and the Scorpion were evidently 
so denominated from their likeness to these animals. 
But, in most cases, it is impossible for us to find any 
reason for designating a constellation by the figure of 
the animal or hero which is employed to represent it. 
These representations were probably once connected 
with the fables of heathen mythology. The same fig- 
ures, absurd as they appear, are still retained for the 
convenience of reference ; since it is easy to find any 
particular star, by specifying the part of the figure to 
which it belongs ; as when we say a star is in the 
neck of Taurus, in the knee of Hercules, or in the tail 
of the Great Bear. This method furnishes a general 
clew to their position ; but the stars belonging to any 
individual constellation, are distinguished according 
to their apparent magnitudes, as follows : First, by the 
Greek letters, Alpha, Beta, Gamma, &c. Thus, Alpha, 
of Orion, denotes the largest star in that constellation ; 

How many stars of the first magnitude 1 How many of the second 1 
Of the third 1 How many within the first seven 1 

305. What is said of the antiquity of the constellations'? Origin 
of their names 1 Why are the ancient figures retained 1 How are 
the individual stars of a constellation denoted 1 


Beta, of Andromeda, the second star in that; and 
Gamma, of the Lion, the third brightest star in the 
Lion. When the number of the Greek letters is insuf- 
ficient, recourse is had to the letters of the Roman 
alphabet, a, b, c, &c. ; and in all cases where these 
are exhausted, the final resort is to numbers. This 
will evidently at length become necessary, since the 
largest constellations contain many hundreds or even 
thousands of stars. 

306. When we look at the firmament on a clear 
Autumnal or Winter evening, it appears so thickly set 
with stars, that one would perhaps imagine, that the 
task of learning even the brightest of them would be 
almost hopeless. So far is this from the truth, that it 
is a very easy task to become acquainted with the 
names and positions of the stars of the first magnitude, 
and of the leading constellations. It is 'but, at first, 
to obtain the assistance of an instructor, or some friend 
who is familiar with the stars, just to point out a few 
of the most conspicuous constellations. A few of the 
largest stars in it will serve to distinguish a constella- 
tion, and enable us to recognise it. These we may 
learn first, and afterward fill up the group by finding 
its smaller members. Thus we may at first content 
ourselves with learning to recognise the Great Bear, by 
the seven bright stars called the Dipper; and we might 
afterward return to this constellation, and learn to 
trace out the head, the feet, and other parts of the ani- 
mal. Having learned to recognise the most noted of 
the constellations, so as to know them the instant we 
see them anywhere in the sky, we may then learn 
the names and positions of a few single stars of special 
celebrity, as Sirius, (the Dog-Star,) the brightest of all 
the fixed stars, situated in the constellation Canis Ma- 

306. Is it a difficult task to learn the constellations, and the names 
of the largest stars 1 What directions are given 1 


jor, (the Greater Dog;) Aldebaran, in Taurus ; Arc- 
turus, in Bootes ; Antares, in the Scorpion ; Capella, in 
the Wagoner. 

307. It is a pleasant evening recreation for a small 
company of young astronomers to go out together, and 
learn one or two constellations every favorable eve- 
ning, until the whole are mastered. A map of the 
stars, placed where the company can easily resort to 
it, will, by a little practice, enable them to find the 
relative situations of the stars, with as much ease as 
they find those of places on the map of any country. A 
celestial globe, when it can be procured, is better still ; 
for it may be so rectified as to represent the exact 
appearance of the heavens on any particular evening. 
It will be advisable to learn first the constellations of the 
zodiac, which have the same names as the signs of 
the zodiac enumerated in Article 203, (Aries, Taurus, 
Gemini, &c. ;) although any order may be pursued 
that suits the season of the year. The most brilliant 
constellations are in the evening sky in the Winter.* 

308. Great difficulties have attended the attempt to 
measure the distances of the fixed stars. We must 
here call to mind the manner in which the distances 
of nearer bodies, as the moon and the sun, are ascer- 
tained, by means of parallax. The moon, for exam- 
ple, is at the same moment projected on different 
points of the sky, by spectators viewing her at places 
on the earth at a distance from each other. (See 
Art. 213.) By means of this apparent change of place 
in the moon, when viewed from different places, astron- 

* For more particular directions for studying the constellations, inclu- 
ding a description of the most important of them, the author begs leave to 
refcr to his larger books, as the " School Astronomy," and " Letters on As- 

307. What is proposed as an evening's recreation 1 What use is 
to be made of a celestial map or globe 1 With what constellations 
is it advisable to commence 1 

308. What is said of the attempt to measure the distances of the 


omers, as already explained, derive her horizontal par- 
allax, and from that her distance from the center of 
the earth. The stars, however, are so far off, that 
they have no horizontal parallax, but appear always 
in the same direction, whether viewed from one part 
of the earth or another. They have not, indeed, until 
very recently, appeared to have any annual parallax ; 
by which is meant, that they do not shift their places 
in the least in consequence of our viewing them at 
different extremities of the earth's orbit, a distance 
of 190,000,000 of miles. The earth, in its annual 
revolution around the sun, must be so much nearer to 
certain stars that lie on one side of her orbit, than she 
is to the same stars when on the opposite side of her 
orbit ; and yet even this immense change in the place- 
of the spectator, makes no apparent change in the- 
position of the stars of the first magnitude ; which-,, 
from their being so conspicuous, were naturally infer- 
red to be nearest to us. Although this result does not 
tell us how far off the stars actually are, yet it shows-- 
us that they cannot be within a distance of twenty 
millions of millions of miles ; for were they within 
that distance, the nicest observations would detect in 
them some annual parallax. If these conclusions are 
drawn with respect to the largest of the fixed stars> 
which we suppose to be vastly nearer to us than 
those of the smallest magnitude, the idea of distance 
swells upon us when we attempt to estimate the re- 
moteness of the latter. Of some stars it is said, that 
thousands of years would be required for their light to 
travel down to us. 

309. By some recent observations, however, it is 
supposed that the long sought for parallax among the 
fixed stars has been discovered. In the year 1838, 

fixed stars'? Have the stars in general any horizontal parallax? 
What is meant by saying that the stars have no annual parallax 1 
Beyond what distance must the great body of the stars be 1 


Professor Bessel, of Koningsberg, (Prussia,) announced 
the discovery of a parallax in one of the stars of the 
constellation Swan, (61 Cygni,) amounting to about 
one third of a second. This seems, indeed, so small an 
angle, that we might have reason to suspect the reality 
of the deterrriination ; but the most competent judges, 
who have thoroughly examined the process by which 
the discovery was made, give their assent to it. What, 
then, do astronomers understand when they say, that a 
parallax has been discovered in one of the fixed stars, 
amounting to one-third of a second ? They mean that 
the star in question apparently shifts its place in the 
heavens to that amount, when viewed at opposite ex- 
tremities of the earth's orbit ; namely, at points in 
space distant from each other 190,000,000 of miles. 
Let us reflect how small an arc of the heavens is one- 
third of a second ! The angular breadth of the sun is 
but small, yet this is toward six thousand times as 
great as the discovered parallax. On calculating the 
distance of the star from us, by this means, it is found 
to be six hundred and fifty-seven thousand seven 
hundred times ninety-five millions of miles, a dis- 
tance which it would take Jight more than ten years to 

SEC. 2. Of Groups and Varieties of Stars. 

310. Under this head, we may consider Double, 
Temporary, and Variable Stars ; Clusters and Nebu- 
lae. Double Stars are those which appear single to the 
naked eye, but are resolved into two by the telescope ; 
or, if not visible to the naked eye, they are such as, 

309. Give an account of the discovery of the parallax of 61 Cygni. 
How much is it 1 What dp astronomers understand by this 1 How 
much less angular breadth is one-third of a second than the breadth 
of the sun 1 What distance does this imply 1 

310. Enumerate the different groups and varieties of the stars. 




when seen in the telescope, are so close together as to 
be regarded as objects of this class. Sometimes, three 
or more stars are found in this near connection, consti- 
tuting triple or multiple stars. Castor, for example, 
(one of the two bright stars in the constellation Gemi- 
ni,) when seen by the naked eye, appears as a single 
star ; but in a telescope, even of moderate power, it is 
resolved into two. These are nearly of equal size ; 
but, more commonly, one is exceedingly small in com- 
parison with the other, resembling a satellite near its 
primary, although in distance, in light, and in other 
characteristics, each has all the attributes of a star, 
and the combination, therefore, cannot be that of a star 

Fig. 122. 

with a planetary satellite. The diagram shows four 
double stars, as they appear in large telescopes. 

311. A circumstance which has given great interest 
to the double stars is, the recent discovery that some 
of them revolve around each other. Their times of 
revolution are very different, varying in the case of 
those already ascertained, from 43 to 1000 years, or 
more. The revolutions of these stars have revealed to 
us this most interesting fact, that the law of gravitation 

What are double stars'? Give an example in Castor. Why may 
not the smaller star be a planetary satellite 1 


extends to the fixed stars. Before these discoveries, we 
could not decide, except by a feeble analogy, that this 
law extended beyond the bounds of the solar system. 
Indeed, our belief rested more upon our idea of unity 
of design in the works of the Creator, than upon any 
certain proof; but the revolution of one star around 
another, in obedience to forces which are proved to be 
similar to those which govern the solar system, estab- 
lishes the grand conclusion, that the law of gravitation 
is truly the law of the material universe. 

312. Temporary Stars are new stars, which have 
appeared suddenly in the firmament, and after a certain 
interval, as suddenly disappeared, and returned no 
more. It was the appearance of a new star of this 
kind, one hundred and twenty-five years before the 
Christian era, that prompted Hipparchus to draw up a 
catalogue of the stars, so that future astronomers 
might be able to decide the question, whether the starry 
heavens are unchangeable or not. Such, also, was 
the star which suddenly shone out in the year 389, in 
the constellation Eagle, as bright as Venus, and after 
remaining three weeks, disappeared entirely. In 1572, 
a new star suddenly appeared, as bright as Sirius, and 
continued to increase until it surpassed Jupiter when 
brightest, and was visible at mid-day. In a month, it 
began to diminish ; and, in three weeks afterward, it 
entirely disappeared. It is also found that stars are 
now missing, which were inserted in ancient catalogues, 
as then existing in the heavens. 

313. Variable Stars are those which undergo a pe- 
riodical change of brightness. One of these is the 
star Mira, in the whale. It appears once in eleven 

311. What has recently given great interest to the double stars'? 
What inference is made respecting the law of gravitation 1 

312. What are temporary stars 1 What led Hipparchus to num- 
ber the stars 1 What is said of the star of 389 '< Of 1572 1 What 
etars are now missing 1 


months, remains at its greatest brightness about a fort- 
night, being then equal to a star of the second magni- 
tude. It then decreases about three months, until it 
becomes completely invisible, and remains so about 
five months, when it again becomes visible, and con- 
tinues increasing during the remaining three months 
of its period. Another variable star in Perseus, goes 
through a great variety of changes in the course of 
three days. Others require many years to accomplish 
the period of their changes. 

314. Clusters of stars will next claim our attention. 
In various parts of the sky, in a clear night, are seen 
large groups which, either by the naked eye, or by 
the aid of the smallest telescope, are perceived to con- 
sist of a great number of small stars. Such are the 
Pleiades, Coma Berenices, (Berenice's Hair,) and 
Prsesepe, or the Beehive, in Cancer. The Pleiades, 
or Seven Stars, as they are called, in the neck of Tau- 
rus, is the most conspicuous cluster. With the naked 
eye, we do not distinguish more than six stars in this 
group ; but the telescope exhibits fifty or sixty stars, 
crowded together, and apparently separated from the 
other parts of the starry heavens. Berenice's Hair, 
which may be seen in the summer sky in the west, a 
little westward of Arcturus, has fewer stars, but they 
are of a larger class than those which compose the 
Pleiades. The Beehive, or Nebula of Cancer, as it is 
called, is one of the finest objects of this kind for a 
small telescope. A common spy-glass, indeed, is suf- 
ficient to resolve it into separate stars. It is easily 
found, appearing to the naked eye somewhat hazy, 
like a comet, the stars being so near together that their 
light becomes blended. A reference to a celestial 
map or globe will show its exact position in the con- 

313. What are variable stars ! Give an example in Mira, and in 

314. What is said of clusters of stars 1 Give examples. What is 


Stellation Cancer, and it will well repay those who can 
command a telescope of any size, for the trouble of 
looking it up. A similar cluster in the sword handle 
of Perseus, near the well-known object, Cassiopea's 
Chair, in the northern sky, also presents a very beau- 
tiful appearance to the telescope. 

315. Nebula are faint, misty appearances, which 
are dimly seen among the stars, resembling comets, or 
a speck of fog. A few are visible to the naked eye ; 
one, especially, in the girdle of the constellation 
Andromeda, which has often been reported as a newly 
discovered comet. The greater part, however, are 
visible only to telescopes of greater or less power. 
They are usually resolved by the telescope into 
myriads of small stars ; though, in some instances, no 
powers of the telescope have been found sufficient to 
resolve them. The Galaxy, or Milky Way, presents 
a continual succession of large nebulae. The great 
English astronomer, Sir William Herschel, has given 
catalogues of 2,000 nebulae, and has shown that 
nebulous matter is distributed through the immensity 
of space in quantities inconceivably great, and in 
separate parcels of all shapes and sizes, and of all 
-degrees of brightness, between a mere milky veil and 
the condensed light of a fixed star. In fact, more 
distinct nebulae have been hunted out by the aid of 
telescopes, than the whole number of stars visible to the 
naked eye in a clear winter's night. Their appearances 
are extremely diversified. In many of them we can 
easily distinguish the individual stars ; in those 
apparently more remote, the interval between the stars 
diminishes, until it becomes quite imperceptible ; and 

said of the Pleiades'? What of Berenice's Hair'? What of the Bee- 
hive 1 Of the cluster in Perseus'? 

315. What are Nebula? 1 Are any visible to the naked eye 1 How 
do they appear by the telescope 1 What is said of the Galaxy or 
Milky Way 1 How many nebulas did Herschel discover 1 Can we 


in their faintest aspect they dwindle to points so minute, 
as to be appropriately called star dust. Beyond this, no 
stars are distinctly visible, but only streaks or patches 
of milky light. In objects so distant as these assem- 
blages of stars, any apparent interval between them 
must imply an immense distance ; and were we to take 
our station in the midst of them, a firmament would 
expand itself over our heads like that of our evening 
sky, only a thousand times more rich and splendid ; 
and were we to take our view from such a distant part 
of the universe, it is thought by astronomers that our 
own starry heavens would all melt together into the 
same soft and mysterious light, and be seen as a faint 
nebula on the utmost verge of creation. 

316. Many of the nebulae exhibit a tendency toward 
a globular form, and indicate a rapid condensation 
toward the center. These wonderful objects, however, 
are not confined to any particular form, but exhibit 
great varieties of figure. Sometimes they appear of 
an oval form ; sometimes they are shaped like a fan ; 
and the unresolvable kind often assume the most 
fantastic forms. But, since objects of this kind must 
be seen before they can be fully understood, it is hoped 
the learner will avail himself of any opportunity he may 
have to contemplate them through the telescope. Some 
of them are of astonishing dimensions. It is but little 
to say of many a nebula, that it would more than cover 
the whole solar system, embracing within it the immense 
orbit of Uranus. 

SEC. 3. Of the Nature of the Stars, and the System 
of the World. 

resolve them all into stars "? If we were to take our position in the 
midst of a great nebula, what should we see over our heads 1 How 
would our firmament appear 1 

316. What is said of the different forms of nebulae 1 What of 
their dimensions 1 


317. We have seen that the stars are so distant, that 
not only would the earth dwindle to a point, and entirely 
vanish as seen from the nearest of them, but that the sun 
itself would appear only as a distant star, less brilliant 
than many of the stars appear to us. The diameter of 
the orbit of Uranus, which is about 3600,000,000 of 
miles, would, as seen from the nearest star, appear so 
small that the finest hair would more than cover 
it. The telescope itself, seems to lose all power when 
applied to measure the magnitudes of the stars ; for 
although it may greatly increase their light, so as to 
make them dazzle the eye like the sun, yet it makes 
them no larger. They are still shining points. We 
may bring them, in effect, 6000 times nearer, and yet 
they are still too distant to appear otherwise than points. 
It would, therefore, seem fruitless to inquire into the 
nature of bodies so far from us, and which reveal 
themselves to us only as shining points in space. Still 
there are a few very satisfactory inferences that can be 
made out respecting them. 

318. First, the fixed stars are bodies greater than our 
earth. Were the stars no larger than the earth, it 
would follow, on optical principles, that they could not 
be seen at such a distance as they are. Attempts have 
been made to estimate the comparative magnitudes of 
the brightest of the fixed stars, from the light which 
they afford. Knowing the rate at which the intensity 
of light decreases as the distance increases, we can 
find how far the sun must be removed from us, in order 
to appear no brighter than Sirius. The distance is 
found to be 140,000 times its present distance. But 
Sirius is more than 200,000 times as far off as the 
sun ; hence it is inferred, that it must, upon the lowest 
estimate, give out twice as much light as the sun ; or 

317. How would our sun appear from the nearest fixed star 1 
How broad would the orbit of Uranus appear *? 

318. What is said of the size of the stars 1 Are the stars of various 


that, in point of splendor, Sirius must be at least equal 
to two suns. Indeed, it is thought that its light equals 
that of fourteen suns. There is reason, however, to 
believe, that the stars are actually of various magni- 
tudes, and that their apparent difference is not owing, as 
some have supposed, merely to their different distances. 
The two members of the double star in the Swan, (61 
Cygni,) the motion of one of which has led to the 
discovery of a parallax, (see Art. 309,) are severally 
thought to have less than half the quantity of matter in 
the sun, which accounts for their appearing so diminutive 
in size, while they are apparently so much nearer to us 
than the great body of the stars. 

319. Secondly, the fixed stars are Suns. It is inferred 
that they shine by their own light, and not like the 
planets, by reflected light, since reflected light would 
be too feeble to render them visible at such a distance. 
Moreover, it can be ascertained by applying certain 
tests to light itself, whether it is direct or reflected 
light ; and the light of the stars, when thus examined, 
proves to be direct. Since, then, the stars are large 
bodies like the sun ; since they are immensely farther 
off than the farthest planet ; since they shine by their 
own light ; and, in short, since their appearance is, 
in all respects, the same as the sun would exhibit if 
removed to the region of the stars, the conclusion is 
unavoidable that the stars are suns. We are justified, 
therefore, by sound analogy, in concluding that the 
stars were made for the same end as the sun ; namely, 
as the centers of attraction to other planetary worlds, 
to which they severally afford light and heat. The 
chief purpose of the stars could not have been 
to adorn the firmament, or to give light by night, 
since by far the greater part of them are invisible to 

magnitudes 1 How large are the two members of the double star 
61 Cygni 1 
819. How is it shown that the stars are suns 1 For what were they 


the naked eye ; nor as landmarks to the navigator, for 
only a small portion of them are adapted to this pur- 
pose : nor, finally, to influence the earth by their at- 
tractions, since their distance renders such an effect 
entirely insensible. If they are suns, and if they ex- 
ert no important agencies upon our world, but are 
bodies evidently adapted to the same purpose as our 
sun, then it is as rational to suppose that they were 
made to give light and heat, as that the eye was made 
for seeing and the ear for hearing. 

320. We are thus irresistibly led to the conclusion, 
that each star is a world within itself, a sun, attend- 
ed, like our sun, by planets to which it dispenses light 
and heat, and whose motions it controls by its attrac- 
tion. Moreover, since we see all things on earth con- 
trived in reference to the sustenance, safety, and hap- 
piness of man, the light for his eyes, the air for his 
lungs, the heat to warm him, and to perform his labors 
by its mechanical and chemical agencies ; since we see 
the earth yielding her flowers and fruits for his sup- 
port, and the waters flowing to quench his thirst, or to 
bear his ships, and all the animal tribes subjected to 
his dominion ; and, finally, since we see the sun him- 
self endued with such powers, and placed at just such 
a distance from him, as to secure his safety and min- 
ister in the highest possible degree to his happiness ; 
we are left in no doubt that this world was made for 
the dwelling place of man. But, on looking upward 
at the other planets, when we see other worlds resem- 
bling this in many respects, enlightened and regulated 
by the same sun, several of them much larger than the 
earth, furnishing a more ample space for intelligent 
beings, and fitted up with a greater number of moons 

made 1 Might it not have been to give light by night to afibrd land- 
marks to the navigator or to exert a power of attraction on the earth 1 
320. To what conclusion are we thus led 1 For what end were 
the stars made 1 


to give them light by night, we can hardly resist the 
conclusion that they, too, are intended as the abodes of 
intelligent, conscious beings, and are not mere solitary 
wastes. Finally, the same train of reasoning conducts 
us to the conclusion, that each star is a solar system, 
and that the universe is composed of worlds inhabited 
by different orders of intelligent beings. 

321. It only remains to inquire respecting the Sys- 
tem of the World, or to see in what order the various 
bodies that compose the universe are arranged. One 
thing is apparent to all who have studied the laws of 
nature, that great uniformity of plan attends every 
department of the works of creation. A drop of water 
has the same constitution as the ocean ; a nut-shell of 
air, the same as the whole atmosphere. The nests 
and the eggs of a particular species of birds are the 
same in all ages ; the anatomy of man is so uniform, 
that the mechanism of one body is that of the race. 
A similar uniformity pervades the mechanism of the 
heavens. To begin with the bodies nearest to us, we 
see the earth attended by a satellite, the moon, that 
revolves about her in exact obedience to the law of 
universal gravitation. Since the discovery of the tel- 
escope has enabled us to see into the mechanism of 
the other planets, we see that Jupiter, Saturn, and 
Uranus, have each a more numerous retinue, but all 
still fashioned according to the same model, and obe- 
dient to the same law. The recent discovery of the 
revolution of one member of a double star around the 
other, shows that the same organization extends to the 
stars ; and certain motions of our own sun and his 
attendant worlds, indicate that our system is likewise 
slowly revolving around some other system. In each 
of the clusters of stars and nebulae, we also see a mul- 

321. What is said of the uniformity of plan visible in the works ot 
nature 1 Show that a similar uniformity prevails in the general plan 
of the celestial bodies. How is this exemplified in the systemsof Ju- 
piter, Saturn, and Uranus 1 In the revolutions of double stars 1 What 


titude of stars assembled together into one group ; and, 
although we have not yet been able to detect a common 
system of motions of revolution among them, and on 
account of their immense distance, particularly of the 
nebulae, perhaps we never shall be able, yet this very 
grouping indicates a mutual relation, and the symmet- 
rical forms which many of them exhibit, prove an or- 
ganization for some common end. Now such is the 
uniformity of the plan of creation, that where we have 
discovered what the plan is in the objects nearest to us, 
we may justly infer that it is the same in similar ob- 
jects, however remote. Upon the strength of a sound 
analogy, therefore, we infer revolutions of the bodies 
composing the most distant nebulae, similar to those 
which we see prevail among all nearer worlds. 

322. This argument is strengthened and its truth 
rendered almost necessary, by the fact that without 
such motions of revolution, the various bodies of the 
universe would have a tendency to fall into disorder 
and ruin. By their mutual attractions, they would all 
tend directly toward each other, moving at first, in- 
deed, with extreme slowness, but in the lapse of ages, 
with accelerated velocity, until they finally rushed to- 
gether in the common center of gravity. We can con- 
ceive of no way in which such a consequence could 
be avoided, except that by which it is obviated in the 
systems which are subject to our observation, namely, by 
a projectile force impressed, upon each body, which 
makes it constantly tend to move directly forward in a 
straight line, but which, when combined with the force 
of gravity existing mutually in all the bodies of the 
system, gives them harmonious revolutions around 
each other. 

indications of systematic arrangement do we see in the clusters and 
nebulae 1 

322. What would happen to the various bodies in the universe with- 
out such revolutions! How could such a consequence be avoided 1 


323. We see, then, in the subordinate members of 
the solar system, in the earth and its moon, in Jupiter, 
Saturn, and Uranus, with their moons, a type of the 
mechanism of the world, and we conclude that the 
material universe is one great system ; that the combi- 
nation of planets with their satellites, constitutes the 
first or lowest order of worlds ; that, next to these, 
planets are linked to suns ; that these are bound to other 
suns, composing a still higher order in the scale of 
being; and, finally, that all the different systems of 
worlds move around their common center of gravity. 

324. The view which the foregoing considerations 
present to us of the grandeur of the material universe, 
is almost overwhelming; and we can hardly avoid 
joining in the exclamations that have been uttered, 
after the same survey, upon the insignificant place 
which we occupy in the scale of being, nor cease to 
wonder, with Addison, that we are not lost among the 
infinitude of the works of God. It is cause of devout 
thankfulness, however, that omniscience and benevo- 
lence are at the helm of the universe ; that the same 
hand which fashioned these innumerable worlds, and 
put them in motion, still directs them in their least as 
well as in their greatest phenomena ; and that, if such 
a view as we have taken of the power of the Creator, 
fills us with awe and fear, the displays of car6 mani- 
fested in all his works for each of the lowest of his 
creatures, no less than for worlds and systems of worlds, 
should conspire with what, we know of his works of 
Providence and Grace^ to fill us with love and adora- 

323. Describe J;he system of the world. 

324. What is said of the grandeur of these views 7 What is spe- 
cial cause of thankfulness 1 How should the contemplation of the 
subject affect us 1 , 

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